Healpful Tips

Aquarium Water Quality & Parameters

Creating and then maintaining the correct water chemistry for the type of fish, as well as the invertebrates and plants you desire to keep are factors that are fundamental to maintaining a healthy aquarium habitat. In order to do this in the most effective manner possible, you may find it useful to understand something of the more technical aspects of the many terms that are used to describe the equipments and properties of water in the aquatic systems. While some of these terms are more appropriate to the description of stream water, lakes, or ponds, all are useful in describing changes which develop in an aquarium environment.



A substance which releases hydrogen ions (H+) when it is added to water .The hydrogen ions is solvated, that is, water molecule adds on to it, to give the oxonium ion (H3O+). Acidic solutions have a pH of less than 7, and have excess hydrogen ions in solution.

Acclimation/Acclimatisation: The process in which the water an organism comes in is balanced to the water in the display aquarium the organism will be going into.

Acrylic is a clear plastic material that resembles glass, but has properties that make it superior to glass in many ways. Acrylic is used to construct aquariums, it is half the weight of glass and its refractive index is closer to that of water than glass, making it more transparent. Common brands of high-grade acrylic include Polycast, Lucite and Plexiglas.

Swelling at the tip of a tentacle that is full of nematocysts.

Actinic Lights
This light produces an identifiable change when it interacts with matter. The light wavelength promotes the growth of Zooxanthellae algae, essential for the growth and well-being of all photosynthetic corals and invertebrates.

Activated carbon
Material (charcoal) used in mechanical/chemical filtration systems to remove, by adsorption, dissolved matter.

Adipose fin
The usually very small fin between the dorsal fin and the caudal fin.

Requires the presence of oxygen to live.

Aerobic Bacteria
Bacteria that requires oxygen to survive.

Pertains to corals that do not have zooxanthellae, generally non-reef building coral.

A group of aquatic, photosynthetic, eukaryotic organisms ranging from microscopic unicellular to multicellular forms, and generally possess chlorophyll but lack true roots, stems and leaves characteristic of terrestrial plants. Algae produce oxygen (photosynthesize) with sunlight (natural or artificial) and use oxygen (respire) during the night hours. Some types of algae are beneficial to aquatic environments while others can be considered a nuisance.

Algal Bloom
An explosive growth of algae which discolours the water and can lead to oxygen depletion.

A chemical that will kill algae but it may also kill other desirable life forms. Care must be taken if you use an algaecide. This is simply treating the symptoms of a larger problem. Find out why you have an algae problem in the first place.

The measure of the number of hydrogen ions in water, expressed in terms of pH; solutions above pH 7 are alkaline. A secondary meaning of alkalinity describes the level of carbonate hardness of water.

The capacity of a system to resist a downward change in pH. Also referred to as carbonate hardness. The recommended range is 8-10 dkh.

The stunting of growth or inhibition of growth of aquatic animals by means of chemical warfare. This occurs in aquariums between competing animals of the same or different species.

Ammonia (NH3)
A highly soluble gas and the main nitrogenous excretory product of most fishes, it is highly toxic to fish and invertebrates. Ammonia can also be added to public water sources to bond with chlorine to form chloramines.

Ammonium (NH4-)
A molecule of ammonia plus an additional hydrogen ion and charge. This ionized form of ammonia is fairly non-toxic to fish.

Any member of the invertebrate order Amphipoda (class Crustacea) inhabiting all parts of the sea, lakes, rivers, sand beaches, caves, and moist (warm) habitats on many tropical islands. Marine amphipods have been found at depths of more than 9,100 m (30,000 feet). Freshwater and marine beach species are commonly known as scuds; those that occupy sand beaches are called sand hoppers, side swimmers or sand fleas (see sand flea). About 6,000 species have been described.

The saltwater version of the parasitic single cell organism in “velvet disease”.

Anaerobic Bacteria
Bacteria that survive in conditions void of oxygen.

Anal Fin
Fin just behind anal opening.

Is the study of internal structures of living organisms.
A general term used by aquarists (sometimes) to describe a place in the marine aquarium when oxygen is in short supply – not void of oxygen; just low in oxygen – enough so that denitrification may occur.

The study of fish health and the management of fish health in aquariums.

The interior artistic arrangements of rocks and corals to create a harmonious and aesthetically pleasing display in an aquarium.

A calcium based sand or substrate used in saltwater aquariums.

Also known as brine shrimp.  They are small crustaceans that live in brine or brackish water.  Artemia nauplii or baby brine shrimp are used mainly for rearing fry.  Adult brine shrimp are used for feeding fish. Artemis is available live, frozen and freeze dried.

Arthropods are invertebrate animals having an exoskeleton (external skeleton), a segmented body, and jointed appendages. They are members of the Phylum arthropoda, and include the insects, arachnids, crustaceans, and others. Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of a-chitin; the cuticles of crustaceans are also bio-mineralised with calcium carbonate.

Artificial Reef
An artificial reef is a man-made, underwater structure, typically built for the purpose of promoting marine life in areas of generally featureless bottom. Artificial reefs may also serve to improve hydrodynamics for surfing or to control beach erosion.

Having no sexual organs, being neither male nor female.

Asexual Reproduction
Reproduction in which it is not necessary to have two parents to produce offspring. The reproduction of single-celled organisms through fission, and the production of spores in some plants and plant like organisms, is examples of asexual reproduction. Some corals reproduce asexually.
The spontaneous casting off of a limb or other body part. Normally occurring when an organism is injured or under attack.

An organism capable of synthesizing (making) its own food from inorganic substances. These organisms use light or chemical energy.


Referring to micro organisms found in aquariums and ponds, some are beneficial such as Nitrosamines and Nitrobacteria in breaking down fish waster from toxic ammonium into nitrite into less harmful nitrate. Other bacteria are harmful and act as pathogens attacking primarily weakened fish and causing diseases.

Is the study of bacteria.                         

An electrical device for starting and regulating fluorescent and discharge lamps.

Base Rock
A generic term for aragonite rocks which have no bacterial organisms or coralline algae growing in or on the rock. Base rocks are often used to build height and depth in reef tank set-ups.

Relating to the bottom under a body of water (the seafloor or bottom of a lake for example) where flora and fauna are found.

Berlin System
A filtration technique for reef aquariums developed in Berlin. The Berlin system relies on live rock and a powerful protein skimmer for water circulation and filtration.


Compound which contains HCO3‑1 ion; mildly basic, example: sodium bicarbonate (NaHCO3).

A biological media typically used in many wet/dry type filters. Generally made of plastic in a spherical shape with a large surface for beneficial bacteria to colonize.
Biochemical oxygen demand (BOD)
This is a measure of the amount of oxygen consumed by the biological process of respiration in the aquarium. Tests are used to monitor water pollution by organic substances such as the waste products of fish because decomposition of organic materials in a closed aquatic system utilizes oxygen.
Biodiversity is the variation of life forms within a given ecosystem, biome, or on the entire Earth. Biodiversity is often used as a measure of the health of biological systems. The biodiversity found on Earth today consists of many millions of distinct biological species.

A slimy matrix produced and inhabited by bacteria which enables the bacteria to adhere to a surface and carry out certain biochemical processes essential to the Nitrogen Cycle. In open aquatic environments, biofilm supports a microscopic community of various species of bacteria including nitrifying bacteria, algae, protozoa, and microscopic invertebrates. In the aquarium, biofilm covers all surfaces exposed to water, and tends to build up in the dark, undisturbed areas such as the filter intake tube and other filter parts.

Biological indicators are species used to monitor the health of an environment or ecosystem. They are any biological species or group of species whose function, population, or status can be used to determine ecosystem or environmental integrity.


Biological Filtration
Means of water filtration using of media to grow colonies of Nitrosomonas and Nitrobacter bacteria to facilitate the reduction of dissolved toxic waste in the form of ammonia and nitrite and Conversion to safer substances such as nitrates.

The study of living organisms, divided into many specialized fields that cover their morphology, physiology, anatomy, behaviour, origin, and distribution.

Animals and microbes those are able to produce their own light. An example would be squid changing colour, or copepods changing colour to thwart potential predators.

Refers to the total mass of living matter within a specified area. Also sometimes referred to as bioload.

Refers to a filter that contains both mechanical and biological filtration all in one unit, or at least has the option to portray both capabilities.  
A natural region or geographical space that presents relative uniformity of physical characteristics and animal/plant populations which inhabit it.

Biotope Aquarium
An aquarium that is designed with the intent to reproduce an existing natural habitat or niche, in terms of its physical/visual characteristics and its naturally occurring species.

The process in which zooxanthellae expel themselves from the tissue of the corals they are living in causing a lack of pigmentation in the coral.  Usually associated with high water temperatures or because of excessive lighting.

A pink to reddish cast on the fins and tail of a fish indicating stress, often caused by ammonia or nitrite poisoning.
Is the study of plants.
Brackish water
Brackish water has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. Brackish water contains between 0.5 and 30 grams of salt per litre, more often expressed as 0.5 to 30 parts per thousand (ppt or %).   
Brine shrimp
Scientific name is Artemia salina, saltwater crustacean, whose dry-stored eggs can be hatched to provide live food for fish and invertebrates.

Bristle worms
Polychaete worms that primarily consuming detritus.  Once thought to be harmful to reef aquariums, most now believe they are beneficial to a reef tank.

A form of asexual reproduction where an organism creates a new individual from an outgrowth capable of surviving on its own.
Chemical compound that moderates and controls the levels of acid in solution, prevents sudden changes in pH upon the addition of either a strong acid or base.

The ability of water to resist changes in pH. Also, used to describe situations when additives are placed into the aquarium.
Buffering capacity
Total amount of all chemical compounds capable of absorbing acids; an adequate amount in water stabilises pH, allows for accurate pH measurements. This also refers to as (Total Alkalinity) its ability to resist pH changes.


Any substance formed of, or containing, calcium carbonate (CaCO3), a substance which may help to maintain a high pH of the aquarium water.

The process by which organic tissue becomes hardened by a deposit of calcium salts within its substance.

A major trace element found in natural and synthetic seawater.  Corals in reef tanks will utilize calcium to build their skeletons. Calcium needs to be replenished in a closed system (Aquarium).
Calcium Carbonate
Calcium carbonate, or CaCO3, comprises more than 4% of the earth’s crust and is found throughout the world, is composed of three elements, which are of particular importance for all organic and inorganic material on our planet: carbon, oxygen and calcium. Calcium carbonate (CaCO3) is a white solid, is non-toxic and odourless. It’s most common natural forms are chalk, limestone, and marble, produced by the sedimentation of the shells of small fossilized snails, shellfish, and coral over millions of years.

Calcium chloride
CaCl2. This is a form of calcium that may be added to reef tanks to maintain the calcium level. However, kalkwasser (calcium hydroxide) is preferred, as adding kalkwasser does not upset the alkalinity or ionic balance the way calcium chloride can.

Calcium hydroxide
Ca(OH)2. See kalkwasser.

Calcium Reactor
A device used to create a balance of alkalinity in the system. An acidic solutions produced by injecting carbon dioxide into a chamber with salt water and calcium-rich media. The carbon dioxide lowers the pH, dissolving the calcium media, and producing a solution high in carboxylic acid. The effluent is then returned to the reef aquarium where the calcium is consumed by organisms, primarily corals when building skeletons. A calcium reactor is one of the most efficient methods of supplying calcium to a reef aquarium.

A mineral form of calcium carbonate (CaCO3) which is found in limestone.

Captive Bred
Captive bred are the offspring of species which were themselves born or hatched in captivity. It may also refer to species bred in captivity from wild caught parents. Captive bred species are generally healthier and parasite free and acclimatise well in captive life. They will generally readily feed on easily obtainable food (such as frozen or other prepared food). Captive breeding is the process of breeding species in human controlled environments with restricted settings, such as wildlife preserves, zoos, conservation facilities, and commercial breeding farms; sometimes the process is construed to include release of individual organisms to the wild, when there is sufficient natural habitat to support new individuals or when the threat to the species in the wild is lessened. This helps to ensure survival of species that would otherwise go extinct in the wild.

Captive Environment
Refers to the "new home" provided to species in captivity, under human care. Captivity can be used as a generalising term to describe the keeping of either domesticated species or wild caught. This may include for example farms, private homes and zoos. A good captive environment is one where animals can acquire and retain the behavioural skills they would need to cope successfully with their natural environment where they ever to be released in the wild. This will allow them to develop and display the skills they would need for survival.

A bony or hard shell that covers part or all of an animal, such as turtles, crabs, and boxfish.


Carbon (Activated)
A chemical filter media used to adsorb odours, dissolved organics, pollutants, medications and discolorations.
Carbon dioxide (CO2)
Non-combustible product of respiration; slightly acidic, water soluble gas; when dissolved in water forms carbonic acid, and used by aquarium plants in the process of photosynthesis.

Compound which contains the CO3-2 ion; i.e.: sodium carbonate (Na2CO3) calcium carbonate (CaCO3) dolomite, limestone and crushed coral.

A hard outer covering or shell made of bone or chitin, such as the fused dorsal plates of turtles.  Another example is the portion of an exoskeleton covering the head and thorax of a crustacean. 

Carbonate Hardness
See alkalinity.
Meat eater.

Caudal Fin
The "tail fin." In many species this is the main fin associated with propulsion.

Water running in limited paths and avoiding most of the filter media due to clogging by particulate matter.
Chemical Filtration
A method of filtering using media, most often activated carbon, to remove unwanted substances in water.

Chemical oxygen demand (COD)
The chemical oxygen demand is measure of organic matter in water and provides a good estimate of the BOD.

A water-cooling mechanism that utilizes refrigerant to cool aquarium water as it passes through the unit.  Mainly used in reef aquariums, Chillers can also be used in freshwater and saltwater fish only (FOWLR) systems.
Product of the reaction of chlorine with ammonia, most probably NH2Cl; added to tape water to destroy harmful bacteria. It can be toxic to fish.


Highly reactive element usually occurring as a gas (Cl2) may react with many substances to form solids and liquids; added to tap water to destroy harmful bacteria. It can be toxic to fish.     

Animals from the phylum Chordata having at some stage of development a dorsal nerve cord and flexible spinal column (notochord).

Circum Tropical
Dealing with areas around tropical or equatorial areas especially waterways.
Closed Loop
An aquarium system in which all components are connected to each other with no other opening other than the aquarium. Canister filters are an example of a closed loop filter.

Phylum that contains hydras, hydroids, jellyfish, sea anemones and corals.

An aquatic animal of the Phylum Coelenterata which is characterized by a central mouth usually surrounded by tentacles bearing stinging cells, and no anus; includes sea anemones, corals, and jellyfishes.

A stiff gelatinous matrix found on certain soft corals that anchor and supports the polyps, and may be embedded with sclerites or other particulate matter. The tissue of a zoanthid that surrounds the polyps, consists of mesoglea and may have sand imbedded in it.

The soft tissue that lies over the stony skeleton and usually links the tissue between polyps.

Colour Rendition Index (CRI)
Is a measure of the ability of a light source to reproduce the colours of various objects being lit by the source. It is a method devised by the International Commission on Illumination.

One of several types of symbiotic relationships between the individuals of two (or more) different species In a comensal relationship one species benefits while the other does not (see symbiosis).

Refers to a symbiotic relationship in which one species benefits while the other is unaffected.

Community aquaria
An aquarium which is populated with fish of several different species, all of which are compatible with one another.

The measurement of water’s ability to conduct an electrical current, the higher the concentrations of minerals the grater will be the water conductivity.

Conservation is an ethic of resource use, allocation, and protection. Its primary focus is upon maintaining the health of the natural world: its, fisheries, habitats, and biological diversity. Secondary focus is on materials conservation and energy conservation, which are seen as important to protect the natural world. Those who follow the conservation ethic and, especially, those who advocate or work toward conservation goals are termed conservationists.

Refers to animals of the same species.

Electronic monitoring devices that have the ability to power on and off other devices. The most popular types of controllers include pH, temperature and Oxidation Reduction Potential (ORP).
Coral sand
Pulverized coral with a particle size similar to that of sand.

Coral like animals from the order zoantharia, commonly called "mushroom corals/anemones" or "false corals."

Coralline Algae
An encrusting form of algae that forms calcareous crusts like coral. Coralline algae are very colourful, occurring in bright purple, pink and red colours. It is very desirable in the reef tank, and can be made to grow on rocks and other hard surfaces by maintaining optimum pH, alkalinity and calcium levels.

The coral structure or cup formed by an individual polyp in a colony.

An animal that feeds on corals these may include fish, sea stars or molluscs. There are two recognized types; obligate corallivores; those that feed only on corals and facultative corallivores, which feed on corals, algae, sponges, and molluscs.

The entire coral formed by a colony of polyps.


Coriolis Effect
Results from earth's rotation causing freely moving objects to veer toward the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Hidden, concealed or camouflaged, as in the colouring of an animal.
Cryptocaryon irritans is a parasitic infection where white spots appear on the body and fins. Fish will scratch themselves against rocks and breathing may become rapid if gills are affected. Treatment can be done by copper or other anti-parasite remedies, but this is incompatible with inverts. Cleaner shrimps and wrasses will remove the parasites, but may not keep up with a major infestation. Cryptocaryon is often referred to as the marine equivalent of the freshwater white spot disease, Ichthyophthirius, or Ich.
Cured Live Rock
Live rock that has gone through the curing process (see Curing Live Rock).

Curing Live Rock
With regards to marine systems, this is the process to get rid of decomposing and dead organic materials in and on live rock. There is die-off at almost every stage of handling of live rock coming into the aquarium shop. If put directly into an aquarium, it would create a lot of pollution and slow down the maturing process of the aquarium. Thus curing live rock outside of the main aquarium is usually the best course of action. Minor curing in a newly setup (no fish or higher/sensitive inverts in yet) aquarium is okay, but no curing of rock should be done in an established marine system.

A group of arthropods, including barnacles, lobsters, crabs, shrimp and crayfish.

Cyanobacteria are commonly referred to as red slime algae by hobbyists although it is not really an alga. It is a colour of bacteria which obtain their energy through photosynthesis and are a significant component of the marine nitrogen cycle. Poor water quality with excessive nutrients is the usual causes. To combat, do frequent water changes, siphon out detritus, and use a good protein skimmer.
The process of establishing bacteria through nitrogen cycle.  The three components involved to make this happen are ammonia (NH³ or NH³+4), nitrite (NO²), and nitrate (NO³). In general the cycling process usually takes about three weeks, however, each system set-up is different. Factors such as how many fish, other livestock, and the amount of organic matter present in the aquarium can vary the cycling time.


Freshwater crustacean, the water flea, used as food in the marine aquarium. Remember to remove dead specimens if uneaten.

Equipment used to remove ions from a solution. A common variant contains a mixture of cation exchange resin in the acid form and anion exchange resin in the hydroxyl form inside a replaceable cartridge; ions in aqueous solution are exchanged for the elements of water by passing the solution through the mixed resin.

The demersal zone is the part of the sea or ocean (or deep lake) comprising the water column that is near to (and is significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer of the larger profoundly zone.
Demineralised water
Water that has been treated to remove all minerals.

The process of reducing nitrate and nitrite into gaseous nitrogen, which is less accessible to life forms. Denitrification proceeds through some combination of the following steps: nitrate › nitrite › nitric oxide › nitrous oxide › dinitrogen gas and can be expressed as a redox reaction: 2NO3- + 10e- + 12H+ › N2 + 6H2O.

Non-living particulate organic material such as dead plants, organisms or fecal matter. Detritus is normally colonized by communities of microorganisms which act to decompose the matter.

A diatom is any of a class of microscopic one-celled algae having walls of silica consisting of two interlocking valves. In the hobby, they are known to be the organisms commonly forming brown films on aquarium glass or rocks. Diatoms form their shells from silicate, and can be controlled to some degree by preventing the addition of this compound through the use of purified water.

A group of flagellate protists; mostly marine plankton with populations dependent on temperature, salinity, and depth. About half of all dinoflagellates are photosynthetic, and make-up the largest group of eukaryotic algae aside from the diatoms. Dinoflagellates are an important part of the aquatic food chain. Some species, called zooxanthellae play an important part in the biology of coral reefs.

Drip Loop
To arrange an electrical cord so it hangs down well below the outlet before running back up again. Any water running down the cord will drip down from the loop, rather than running uphill to enter the outlet.


A filter designed to remove unwanted nitrates from an aquarium or pond.  Normally accomplished with a slow flow through media with anaerobic bacteria or via a sulfur-based and aragonite media.

The process by which nitrogen and nitrogenous compounds are removed from water.

Dissolved oxygen
The amount of oxygen in the aquarium water is affected by many variables, for example, warmer water will increase the metabolic rate of the fish and hence their demand for oxygen, warmer water will also dissolve less oxygen than colder water. The solubility of oxygen as well as other gasses also decreases with increasing salinity levels. The amount of surface area also affects oxygen exchange between ambient air and water; creation of surface turbulence by aeration or by power heads will maximise this. Oxygen levels are measured in parts per million; a dissolved oxygen level of 7 to 10 ppm is what you should aim for.

An animal that is active during the day, and sleeping during the night.

Accumulated waste material such as fish waste or excess food.

Organisms that consume detritus.

Having digits or finger-like projections.

Abbreviation of Dead on Arrival.

Abbreviation for Dissolved Organic Compounds.

Drip acclimation
A method for acclimating newly acquired specimens to their new environment.  Consists of slowly dripping water from the specimen’s new home into the water they arrived. This reduces stress because of the change in water parameters between the new and old water.

Dorsal Fin
The fin directly on the top of the body (it's the fin that sticks out of the water when you see a shark). Some fish have two dorsal fins one directly behind the other.


Dosing pump
A pump which can supply a very slow drips which is used to add trace elements or make up water for evaporation.

Down-flow systems
Filters in which water enters the top and flows down through media and then exits at the bottom of the system.


Ecological factor
Any variable of the environment that impacts the life of one or more organisms. May be classified into A: climatic, physiographic, edaphic and biotic factors; or B: direct, indirect, and remote factors.

Is the study of the relationships of living things to each other and to their environment.
Egg Layer
Species of fish where the female lays eggs which are externally fertilised by the male. 

Negatively charged particles that surround an atom's nucleus.

Is the study of the formation and development of living things from fertilization to birth as independent organisms.

Refers to species lives in that specific geographical place and nowhere else.

En Masse Spawn
An event where all or mostly all of the local inhabitants are simulated to spawn all at once.

Used to bond rock, plants or corals together.  Most popular types are two part epoxies that harden when mixed together.

The part of the wide lower course of a river where it’s current is met by the tides of the ocean. Depending on the tides (low and high) this area can go from saltwater to brackish to freshwater.

An organ, such as the siphon on a clam that is used for exhalation.

External Pump
A pump that operates outside the aquarium or pond, these pumps are generally non-submersible.  

In biology and ecology, extinction is the end of an organism or group of taxa. A species becomes extinct when the last existing member of that species dies. Extinction therefore becomes a certainty when there are no surviving individuals that are able to reproduce and create a new generation.


In aquarium usage it is to allow an aquarium or pond to run without inhabitants.  Generally, this is done to help kill off any parasites in the aquarium, as there are no hosts for the parasites to live off of.

A taxonomic category of related organisms ranking below an order and above a genus.

Filter Feeder
An organism that strains out nutrients such as plankton, bacteria, or detritus from the water column to feed upon.

Filtration System (Filter)
Devices that will help purify tank or pond water.  The three main components of a filter system will include mechanical (trapping particles), biological (breaking down of waste products such as ammonia and nitrite) and chemical (adsorbing odours and discoloration).

Fin Rot
A condition in which the fins are rotted away giving the appearance of ragged, split, or perforated fins.

Fishless Cycle
To Cycle an aquatic system without live fish, using chemical or household ammonia, or decaying material such as fish food or a piece of shrimp to promote the establishment of nitrifying bacteria (as Biofilm).

Fish Only System
Refers to an aquarium system without invertebrates or other organisms contained in the tank, only fish.

An asexual reproductive method (Division).

The act of a fish rubbing or scratching their bodies typically to relieve themselves of symptoms associated with parasites or stress.

A chemical that causes particles such a single celled algae and silt to clump together for easier removal.

Fluidised Bed
Aquarium fluidised bed filters involve suspending media in a body of flowing water. Fluidised bed filters are ideal for planted aquariums, as they won't dissipate the CO2.

Foam Fractionators
See Protein Skimmer.

Formal Ponds
Ponds designed in shapes such as circles, squares and rectangles. Conforming to specific design ideas.

Saltwater aquarium containing fish and live rock only.

The process of artificial reproduction of corals by breaking, cutting or dividing pieces from the main colony and attaching them to a new base. These pieces eventually become new colonies.  These cuttings are commonly referred to as "frags."    

Cuttings of corals from a main colony of coral for the purpose of creating a new colony.  Frags are often traded and sold between reef hobbyists.

Fish larvae (baby fish).


A reproductive cell having the haploid number of chromosomes, especially a mature sperm or egg capable of fusing with a gamete of the opposite sex to produce the fertilized egg.

A taxonomic category ranking below family and above species.  Generally consisting on a group of species exhibiting similar characteristics.
The membranes through which fish absorb dissolved oxygen from the water during respiration.
Glitter Lines (Shimmer Lines)
Light lines viewed underwater on surfaces (such as the sand on the bottom of the ocean) caused by ripples or waves on the surface of that reflect and refract the light from a single source point (i.e. the sun in the wild or a metal halide bulb in an aquarium).
A Gorgonian is a tropical or subtropical octocoral with upright branchy plant-like or fan-like growths and a skeleton made of a horny organic material.

Gravity Fed
Water flowing into the filter system by the force of gravity and then returned to the aquarium or pond via a pump.

Ground Fault Interrupters (GFI)
It is a device that measures the amperes (number of electrons) travelling through a circuit and quickly opens (breaks) the circuit if there is a small loss or ground fault to prevent you or your livestock from getting shocked.



Halogen lights have a very yellow light not appropriate for aquarium use. Do not confuse these with metal halide lights.

A social group of female animals of the same species accompanied or dominated by one fertile male who denies other males’ access to the group. A single male spawns repeatedly with several females, perhaps up to dozen, and the entire harem occupies the same general area of the reef. The male protects his territory, and its resident females, from other males that may be passing through or that occupy adjacent territories.

Hard/Soft water
Describes the concentration of certain dissolved minerals contained in water. Hard water has high concentrations of dissolved minerals; soft water has low concentrations. Different species of fish have different preferences for water hardness.

The physical environment of a particular species.

The degree of water hardness is directly proportional to the concentrations of mineral ions in the water. Although other mineral ions such as boron, copper, iron, lead, silicon and zinc may be present in trace amounts, calcium and magnesium are the dominant chemicals found in solution. Thus hardness is usually expressed in terms of the amount of calcium carbonate (CaCO3) present in water. Water hardness is expressed as parts per million (ppm). On German scale, which is widely used by aquarists, the term “DH” refers to degree hardness. One DH is equivalent to 17 ppm. A useful classification of “soft” versus “hard” water quality as follow:
Soft: 0 to 75 ppm. Moderately hard: 75 to 150 ppm. Hard: 150 to 300 ppm. Very hard: + 300 ppm. From the aquarist’s perspective, the most immediate practical aspect of water hardness is that softer water has less buffering capacity. This means that pH can drop abruptly in an aquarium filled with soft water, whereas pH tends to be more stable in an aquarium filled with hard water.

Head Height
Calculated in feet, it is the amount of height a pump pushes against gravity through tubing to the highest point of re-entry into the system. This is important to know when determining the correct pump for your water feature. 

Head and Lateral Line Erosion
Also known as hole-in-head disease and lateral line disease. A fish with this condition will develop holes in it’s' head and sometimes along its' lateral line. The main cause is nutritional deficiency, especially vitamin C. Stress and poor water quality also play a role. Untreated cases will cause disfiguring or death. To combat and cure, ensure good water quality and provide vitamin enriched foods, especially vitamin C.

An electronic device that will heat water to a specific temperature.  Best used with a controller. It is best to replace your heater every 2-3 years.

Vegetable and plant eater.

An organism that possesses both male and female sexual organs.

Refers to an organism that cannot synthesize its own food.  It is dependent on complex organic substances for nutrition (heterotrophy).

Horizontal or Cross flow systems
Water flows through a filter, entering one side, across media and exiting the opposite side of the filter unit.

Hydroids are colonies of polyps. Each polyp is similar to a small sea anemone i.e. has its own jelly like body and a mouth surrounded by tentacles. The difference is that individuals are organised to benefit the whole colony. They do this, by being interconnected via a common tube the stolon. The stolon which can be tough and horny, allows the transfer of food between the polyps. Hydroids can be introduced to a home aquarium through live rocks as hitchhikers.

A device for measuring the salinity or specific gravity (S.G) of salt water.
Hydrogen sulphide (H2S)
Foul smelling gas, partially water-soluble; results from decomposition of plant and animal matter in the absence of sufficient oxygen.

Hyperosmotic stress
Occurs when water to exit the cell, resulting in cell shrinkage, which can lead to DNA and protein damage and ultimately cell death. Damage to fish scales and skin can increase the susceptibility to infection and cause excessive uptake of water by freshwater fish or loss of water from marine species (osmotic stress).



A common parasite that fish get especially when stressed.  Visually appear as small salt grains (small white spots) when seen on fish. There are multiple treatment options for this parasite. Generally not deadly to the fish unless it goes untreated.

Is the study of fishes.
Aquatic animals that live in the substrate of a body of water, especially in a soft sea bottom (sand or mud substrate).

Informal Ponds
ponds designed without strict geometric form.

Known as the region between the high tide mark and the low tide mark. Commonly referring to the organisms that dwell in this area.

An animal lacking a backbone or spinal column.
A trace element found in seawater necessary in small quantities for some reef invertebrates, particularly corals and clams.

Any changed chemical species; positive ion = cation; negative ion = anion.

Ion-exchange resin
Material used to remove minerals from the water and hence soften it.


Jaubert System
Named after Dr. Jean Jaubert.  A filtration system/method used in reef aquariums.  It consists of a deep bed of sand over a buried plastic plate.  Beneath the plate is an area void of sand with a pocket of low oxygenated water known as a plenum. Nitrogenous wastes are broken down as the aquarium water diffuses through the area.


Kalkwasser (Lime Water)
A saturated solution of calcium hydroxide Ca(OH)2 used to raise the calcium levels in a reef tank.  Kalkwasser is available as a powder that is mixed with RO or RO/DI water and dripped into the tank. About 1 teaspoon of Ca(OH)2 powder will dissolve in a gallon of pure water.

Temperature scale used to designate light intensity. For freshwater 6400K-10000K is optimum. For marine 6400K – 20000K is optimum.


A thin scale or plate like structure.  Sometimes used to describe flow (laminar flow). Commonly refers to a unidirectional flow in a flat, horizontal plane.

The first stage of development after hatching for many fish, and invertebrates with indirect development, undergoing metamorphosis.

Lateral Line
A fish's sense organ used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the gills to the base of the tail.

LED (Light Emitting Diode)
A diode is the simplest sort of semiconductor device. The semiconductor material for LED's is typically aluminium-gallium-arsenide (AlGaAs). In pure aluminium-gallium-arsenide, all of the atoms bond perfectly to their neighbours, leaving no free electrons (negatively-charged particles) to conduct electric current. Light is produced from energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called photons, are the most basic units of light. Photons are released as a result of the moving electrons.

Live bearers
Fish which produce live young rather than laying eggs.

Live Rock
Rock that has been collected from marine environments used as decoration and filtration in marine fish only and reef tanks.  It will contain micro and macro flora and fauna beneficial to the aquarium.

Live Sand
Live sand, or LS is natural reef coral sand that is collected live from the ocean, or non-living coral sand that is cultured to make it live. What makes it live is the microscopic biological bacteria that grows on it, and the many tiny crustaceans and other micro and macro-organisms that reside in it. Live sand can serve as the main base for biological filtration in a saltwater aquarium, while the organisms help consume organic matter in the sand bed. Some of the organisms provide a natural food source for many aquarium inhabitants as well.

Abbreviation for Large Polyp Stony coral.


Algae, such as seaweeds, that project more than one centimetre above the substratum.


A fossil coral reef.

Large, pigmented fleshy portion of tridacnid clams that is exposed to the light by gaping of the shell valves. Also called siphonal tissue. Also, the coral tissue in fleshy polyps (e.g. Catalaphyllia).

Marginal/Emergent Plants
Plants that grow in or around the edges of the pond. Their leaves and stems rise above the water line and they grow rooted in a submersed soil. Typically their crown is not submerged any more than a couple inches below the water.

Mechanical Filtration
A method of removing solid particles from the aquarium water.

A broad term used by marine aquarists to describe a purposefully placed solid material into the marine system. It includes things like substrates, absorbents, resins, filter material, etc. Examples include but aren’t limited to: activated carbon, sand, bio-balls, resins that absorb wastes/pollutants, etc.


A filter media that will allow certain molecules or ions to pass through it by diffusion. The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute.

Mercury (Hg)
A silvery-white poisonous metallic element, liquid at room temperature and used in fluorescent lamps to furnish mercury vapour.

Metal Halide Lamp
A high-pressure discharge lamp that is enclosed in a quartz tube containing metal halides; usually iodides, and produces high-efficacy light. These lamps are sometimes used in marine aquarium applications. These lamps always require ballast, which supplies the proper voltage and current for starting and operating the lamps.

Various organisms that spend part of their life cycle as plankton.  Most commonly the plankton state occurs during the larval or egg stages.

Microalgae: Refers to alga that is not microscopic. Microalgae are commonly placed in refugiums in saltwater aquariums.  Some examples include Caulerpa and Chaetomorpha.

Is the study of microorganisms.
Microfauna refer to small, mostly microscopic animals, such as protozoa, nematodes, small arthropods, etc. In addition there are other components of the microbiota, such as fungi and bacteria.
Naturally occurring inorganic substance found in water. High levels of dissolved minerals in aquarium water may be harmful to some species, and some minerals at trace levels are important for the health of livestock.

The breakdown by bacteria of nitrogenous waste products.

Electronic devices that are able to read different water parameters such as pH, temperature and ORP.  Monitors differ from controllers because they do not have the ability to turn on or off other devices. Monitors only display the water parameters.

A branch of biology that deals with both the external and internal structure of the organisms without the consideration of its function. Thus, it may be divided into two distinct branches: the anatomy and the eidonomy.
Mobile or movable. Moving or having the power to move spontaneously.

Moulting, also known as sloughing, shedding, or for some species, ecdysis) signifies the manner in which an animal routinely casts off a part of its body (often but not always an outer layer or covering), either at specific times of year, or at specific points in its life-cycle.

An association between two different species of organisms in which each organism benefits.

Is the study of fungi.
mV: (millivolt) a unit of potential equal to one thousandth of a volt.


Resembling mother-of-pearl; lustrous. Usually used to decribe the appearance of pearlscale goldfish.

Nanometre (nm)
A unit in which wavelengths of light are expressed. 1 nm = 0.000000001 meter (1 billionth of a meter).

Nano Plankton
Very small plankton. Composed of organisms measuring from 2 to 20 micrometers.

The larval stage, of a crustacean such as Artemia salina or brine shrimp. "Artemia nauplii" are commonly used as a first food for fish.

The process of tissue dying off. Refers to the death of cells or tissues from injury or disease. Also referred to as Rapid Tissue Necrosis (RTN) or Slow Tissue Necrosis (STN).

A capsule within a specialized cell of certain coelenterates (such as jellyfish) containing barbed, threadlike tube that delivers a paralysing sting when propelled into attackers and prey.  Sometimes also referred to as "stinging cells". Also the cells at the tip of anemones' tentacles which "sting" when touched.
Nitrate (NO3-): Less toxic ammonium compound produced by Nitrobacter bacteria from Nitrite. Nitrate levels can be kept to a minimum by regular partial water s.
Nitrification: The biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. The oxidation of ammonia into nitrite, and the subsequent oxidation to nitrate are performed by two different nitrifying bacteria; Nitrosomonas and Nitrobacter.
Nitrite (NO2)
Toxic ammonium compound produced by Nitrosomonas bacteria from ammonia. It is toxic to fish, and even more so to invertebrates.

Nitrobacter Bacteria
A rod-shaped aerobic bacterium that is an important part of the nitrogen cycle, oxidizing nitrite in aquarium filter water into nitrate.

Nitrogen Cycle
A naturally occurring chemical process by which organic wastes in conjunction with wastes from fish are converted by aerobic bacteria to nitrite, and then in turn are converted again by aerobic bacteria to nitrate.

Nitrosamines Bacteria
A rod-shaped aerobic bacterium that is an important part of the nitrogen cycle, oxidising ammonia in aquarium filter water in to nitrite.

Nitrospiralis Bacteria
Another specific type of aerobic bacteria that converts nitrite to nitrate.


An animal that is active during the night, and sleeping during the day.

A type of dried seaweed used to wrap sushi.  Also used to feed herbivore fish such as Tangs.


Obligate or required - i.e. an obligate parasite is a parasitic organism that cannot live independently of its host, or an obligate aerobe is an organism that cannot survive without oxygen.

A member of the phylum Coelenterate (Cnidarians), class Anthozoa characterized by normally having eight tentacles on each polyp.

Eats all food.

Rock, usually limestone composed of oolites.  A small round calcareous grain found, for example, in limestone.

A “platelike” covering over the gills in fishes to help protect the gills.  

A taxonomic category of organisms ranking above a family and below a class.

Relating to carbon-based plant or animal constituents or products.

Organic load
A term referring to all animal and plant life in a system. Primarily fish and decomposing organic matter.

ORP (Oxidation Reduction Potential)
Measured in millivolts (mV) a measurement of the loss and gain of electrons by a molecule, atom or ion. For aquarium keeping purposes, ORP is the measurement of the "liveability" of the aquarium water. Natural seawater has a range of 350-400 mV; however some aquarists have broken the 400 mV barriers with great success.

The “mouth like” opening in a sponge that is used to expel water.  Also referred to as the osculum.


Having one or more pairs of eyespots.  Theorized as a way to confuse any possible predators.

This is the word used to describe how the fish controls the amount of salt it has in its body. The environment the fish is in is high in salt content. The marine fish needs to obtain fresh (non-salted water) to carry out internal biological functions. The fish drinks salt water, removes the salt, and sends the salt back into its environment. The fish internal system controls the amount of salt inside the fish. This control is known as osmoregulation and it takes quite a bit of the fish’s energy.

Osmoregulatory System
The system that provides blood to carry oxygen throughout the fish’s circulatory system.

Diffusion of fluid through a semi permeable membrane from a solution with a low solute concentration to a solution with a higher solute concentration until there is an equal concentration on both sides of the membrane.

Osmotic Stress
An adverse reaction caused when the salinity of an animal's environment changes drastically.

Osmotic Stress
Occurs when the concentration of molecules in solution outside of the cell is different than the concentration inside the cell. Water flows either into or out of the cell by osmosis, thereby altering the cell environment and causing stress.
A small opening or orifice.  Also refers to any of the small openings or pores in a sponge.

Oviparous animals are animals that lay eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, reptiles, all birds, the monotremes, and most insects and arachnids.

Oxidation Reduction Potential (ORP)
Measured in millivolts (mV), it is the measure of the ability of the water to break down waste products in the tank.  For aquariums the recommended level is between 350-390 mV.
Produced when oxygen (O2) molecules are dissociated by an energy source into oxygen atoms and subsequently collide with an oxygen molecule to form an unstable gas, ozone (O3), which is can be used to disinfect aquarium water. Ozone must be generated in proximity to the aquarium because it is unstable and decomposes to elemental oxygen in a short amount of time after generation. Ozone is a very strong oxidant and virucide and works by destroying cell walls.

Equipment used to generate ozone (O3) by charging the air with a burst of high negative voltage.


Refers to small organisms that use fish as a host organism for survival. Parasites are one of the major causes of disease in aquarium fishes. Micro-parasites are observed with the use of a microscope and include trichodina, costia, ich, etc. Macro-parasites can be seen by the naked eye and include lice, anchor worm, etc. In a parasitic relationship (parasitism) one species benefits, the other is harmed.
Is the study of diseases, generally in animals.
An agent that causes disease, especially a living micro organism such as a bacterium or fungus.

Pectoral Fins
These fins are on the sides of the fish located directly behind the gills. They are often used for more precise manoeuvring. In a few species they are used for propulsion.

Refers to living in the water of the ocean above the bottom. “Ocean going” or "pertaining to the sea".  Pelagic species have the ability to swim around or move in some fashion. "Pelagic" is also used to refer (usually) to eggs that are basically at the mercy of the ocean currents.
Pelvic Fins
Paired fins towards the underside of the fish located directly below the gills. Vary in size and shape greatly. Also known as the ventral fins. Not all marine fish have these.

Term used to measure the relative amounts of hydrogen ions (H+) and hydroxyl ions (OH-) present in water. It is measured on a logarithmic scale ranges from 1 to 14. At pH 7.0 it is neutral since there are an equal number of hydrogen and hydroxyl ions; at pH values less than 7.0 signifies acidic conditions, in which hydrogen ions are more abundant. At pH grater than 7.0 signifies alkaline conditions, in which hydroxyl ions predominate. Sea water has a higher pH than fresh water and should be maintained between (7.9 and 8.4) a falling pH often indicates that the water is aging and that its buffering capabilities is declining.

pH bounce
Sudden fluctuation in pH caused by improper levels of Buffering Capacity (Total Alkalinity).

Pharyngeal teeth (throat teeth)
Tooth-like structures in the back part of the throat derived from gill supports. Primarily used for grinding, may be used to produce sound.

A polyatomic ion with the empirical formula PO4, Phosphate is a prime food source of algae, particularly cyanobacteria or slime algae. Phosphate can cause uncontrolled growth of algae in the aquarium; it can also be toxic in high concentrations and must be kept to a minimum level in reef aquarium.

A substance that exhibits sustained glowing after exposure to energized particles such as electrons. Phosphor is used to coat the inside of fluorescent bulbs and transforms ultraviolet radiation into visible light. The type of phosphor determines the colour of light output.

An organism, typically a plant, obtaining energy from sunlight as its source of energy to convert inorganic materials into organic materials for use in cellular functions such as biosynthesis and respiration. In order to capture light as source of energy, photoautotroph’s carry out photosynthesis, converting energy from sunlight, carbon dioxide and water into organic materials. Photoautotrophs provide nutrition for many forms of life. They include the plants, algae and certain protists bacteria.

The duration of an organism's daily exposure to light, considered especially with regard to the effect of the exposure on growth and development. In the aquarium hobby, it also refers to the length of time that the aquarium lights remain on, to accommodate certain organisms' requirements to thrive.

Is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria. Photosynthetic organisms are called photoautotroph, since they can create their own food. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is vital for life on Earth.

Photosynthetic Active Radiation (PAR)
Refers to the readings or light levels and energy given off by a lamp.
Is the study of algae.
A primary division of a kingdom, as of the animal kingdom. Ranking next above a class in size.

Is the study of normal functions of living organisms.  

Phyto Filtration
The use of plants to remove excess nutrients from the water and aid in particulate settlement; also referred to as vegetative filtration.

is the study of diseases in plants.
Microscopic free-floating aquatic plants, mainly algae. It lives suspended in bodies of water column that is penetrated by light and drifts about.

Feather like. The 'pinnules' which are found on many octocorals are small side branches of the polyp tentacle which give it a 'pinnate' appearance.

Lateral branches (feather like or plume like) on the tentacles of a soft coral that give it a feather-like appearance.

Pipe Run
Typically calculated in feet, the amount of pipe that a pump is pushing water through to its eventual destination.  Helpful when used in conjunction with head height. This is used to determine the correct pump for your water feature or pond.

Fish-Like or fish. Relating to or characteristics of fish or fishes.

Habitually feeding on fish; eats other fish.

A general term to describe an organism adapted to feed and survive from eating mostly plankton.
Small or microscopic organisms, including algae and protozoans that float or drift in great numbers that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. In aquaria, often found at or near the surface. They provide a crucial source of food to larger, more familiar aquatic organisms such as fish and cetacean.
Planktonic Algae
Single celled, free-floating algae that can make the water appear thick and green like pea soup.

The flat, free-swimming ciliated larva of a coelenterate.

A space between the aquarium bottom and substrate, created by supporting the substrate above the bottom using UGF plates or some other support structure. Used to keep Nitrate levels low in a saltwater aquarium. Water and organic waste diffuses down into the plenum where the wastes concentrate. Nitrifying bacteria in the upper levels of the substrate use the oxygen and produce nitrate in the water as it moves down into the space. Denitrifying bacteria living in the oxygen-poor lower levels of the substrate slowly convert Nitrate into harmless nitrogen gas.

A water pump that is submerged inside the aquarium or pond to add extra water movement within the tank or pond.

The capturing of prey as a means of maintaining life (for nutrition).

Prokaryotes are organisms without a cell nucleus, or indeed any other membrane-bound organelles, in most cases unicellular (in rare cases, multicellular).

The process of multiplication or increasing in numbers. This is done by either natural reproduction or fragmentation.

Protein Skimmer (Foam Fractionators)
A filter that removes organic waste from a saltwater aquarium.  Water and tiny bubbles are mixed within a column.  The organic waste is attracted to the bubbles creating foam (skimmate) that flows into a collection cup.

Any member of the phylum Protozoa, which comprises of unicellular organisms. Protozoans are a major component of the ecosystem. Protozoa is a subkingdom of microorganisms that are classified generally as unicellular non-fungal eukaryotes.

An invertebrate having a hollow, somewhat cylindrical body, attached at one end, with a mouth surrounded by tentacles at the free end. Polyps may be solitary (hydra) or colonial (coral).
Polyvinyl Chloride (PVC)
Hard tubing used for plumbing an aquarium or ponds pumps and/or return water lines.
ppm (parts per million)
Measure of extremely low concentration; 1 ppm equals 0.0001%; 1 ppm equals 1 mg/litre.


Quarantine Tank (Hospital Tank):
A tank set up separately from a main or display system used to house new or sick fish livestock.  Mainly used to prevent spreading diseases into the main established aquarium or pond.  Fish should generally be quarantined for a minimum 2-4 weeks before introduction to the main tank or pond.


A flexible “tongue like” organ in certain molluscs, having a row of horny teeth on the surface.

Rapid Tissue Necrosis (RTN)
Characterized by the rapid loss of tissues on corals generally caused by a pathogen.  Generally will result in the sudden death of corals.

Redox potential
An abbreviation for a body of water’s potential for “reduction” or “oxidation”; water with high redox potential is clear, of high quality and contains much surplus oxygen. A low redox potential indicates water that is of poor quality, and deficient in oxygen.
Red Slime
See Cyanobacteria. To prevent this potential problem from occurring, learn about what phosphate is, where it comes from, and how to reduce or eliminate the accumulation in marine aquariums.

Reverse Osmosis (RO)
A separation process that uses pressure to force water through a membrane that retains the solute on one side and allows the pure water to pass to the other side.

Reverse-flow Filtration
A biological filtration system where water is returned to the aquarium at the base instead of at the water surface.

Reef Tank (System)
A marine aquarium consisting of corals and other invertebrates.  Most will also have "reef safe" fish as well. They require high intensity lighting and very exceptional water quality.

An instrument used to measure salinity by refraction, the amount light deflects from a straight line through a medium.

The part of the marine system where the aquarist has set up to contain marine life forms that wouldn’t survive in the display tank. This is often another tank in the marine system where macro algae, a deep sand bed, mud, and/or other marine organisms, such as copepods, are cultured or maintained.

Refers to the re-growth of a lost appendage or tissue. An example would be a starfish regenerating limbs that have been lost.

The process of obtaining energy from organic material, in effect the opposite of photosynthesis, performed by plants during night time hours with the by-product carbon dioxide.

Reverse Daylight Photosynthesis (RDP)
Term used for running a refugium light opposite of the lights on the main display tank.  This process assists in maintaining stable pH in the aquarium.
Reverse-flow filtration
An arrangement that pumps water through the gravel covering the base of the tank, thereby dislodging any solid waste material, and making it available for collection by the power filter doing the pumping.

Reverse Osmosis or Reverse Osmosis Deionization (RO or RO/DI)
A filtering unit that purifies tap water for use in aquariums.  Using a series of cartridges (sediment, carbon and with deionization) along with a membrane they are able to remove 95 to 99.8% of contaminants found in tap water.

Horizontal underground stems from which roots and shoots develop.


The dissolved salt content in a body of water measured in parts per million (ppm). Typically, freshwater <500 ppm, brackish water 500-30,000 ppm, seawater 30,000 - 50,000 ppm and brine >50,000 ppm. Also see Specific Gravity.

A group of one kind of fish oriented together in a synchronized fashion, with uniform distance between individual fishes, travelling at the same speed and/or oriented in the same direction and position. Please also see "Shoal".

Sessile means the organism is attached to the substrate at the bottom of the ocean and therefore cannot move around.

Sexual Reproduction
A union that results in increasing genetic diversity of the offspring by utilizing chromosomes from two gametes.

A horny, chitinous or bony external plate or scale.  Also called a scutum.

Is loose, hard material lying on top of solid rock. Sediment comes in many forms and sizes, and can originate from a variety of sources. Detrital sediment is produced by the weathering and erosion of rocks exposed at Earth's surface. Chemical sediment forms as minerals crystallize and settle from water that contains lots of dissolved particles. Biochemical sediment accumulates as plants and animals die and their hard parts, such as skeletons and shells, are deposited on the ocean floor.

Refers to animals or organisms that remain or live in one area.  Attached to a surface and not moving freely, such as a barnacle.

The act or process of depositing sediment.

Liquid that is exuded by a squid or octopus called "ink." Composed of a highly concentrated solution of the pigment melanin and mucus and tyrosine.

Permanently attached or fixed, not free-moving.

Settling tanks
Usually conical in shape. They are used before the bio-filtration unit for aquarium systems and settle out solids with slow-moving water. Typically they are gravity fed units.

Sexual Dimorphism
Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. Examples include colour (specifically referred to as sexual dichromatism), size, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers, or tusks.

A group of fish loosely gathered together but with variable distance between individuals and moving in various directions and/or oriented in various positions. Fish that shoal together will often school when on the move or in response to a threat. Please also see "School".

Silicone Sealant
A powerful, flexible rubbery adhesive used in aquarium to bond glasses and many other marine applications to keep water in or out of a designated area. It can also be used in reef aquariums to attach rock and coral formations.

An arrangement whereby water is induced to flow naturally from an upper level to a lower level through a pipe or hose which spans an intermediate level that is higher than either. It also refers to a tubular organ, where aquatic invertebrates such as squids or clams taken in water and expelled out.

A coral's smaller secondary polyp used to move water around the colony.

Slime Coat
The continuously produced mucous envelope which covers the scales of a fish and makes it feel ‘slippery.’ Its purpose is to act as a barrier against disease-causing organisms, to act as a buffer in the internal/external gas exchange systems of the fish, and to save energy by reducing friction with the water when swimming. When a fish is subjected to stress the slime coat becomes thinner.

To shed.

Sodium bicarbonate (NaHCO3)
Mildly basic substance used to increase buffering capacity.

Sodium carbonate (Na2CO3)
Soda ash; crystalline solid, used to raise pH.

Sodium hydrogenphosphate (NaH2PO4)
Crystalline solid, used to lower pH.

A subdivision of a genus containing closely related animals or plants capable of interbreeding. In the nomenclature of plants and fish, the species designation is the second part of the scientific name.

Species tank
A tank containing only a single species of fish.

Specific gravity
Relative density defined as the ratio of the density of a given substance, to the density of water (H2O). Substances with a specific gravity greater than 1 are heavier than water, and those with a specific gravity of less than 1 are lighter than water. S.G is expressed in parts per thousand (ppt). From the aquarist's point of view, salinity is generally determined indirectly by measuring the specific gravity (S.G) using a hydrometers. Most seawater has an S.G. of around 1.024, but Arabian Gulf and Red Sea waters are much denser and have S.G. values of about 1.030 to 1.035.

A skletal element in soft corals (needle like in appearance) composed of calcium carbonate.  Also found in sponges made of silicon dioxide.

Spirulina is a microscopic blue-green alga in the shape of a spiral coil, living both in sea and fresh water. Spirulina is the common name for human and animal food supplements produced primarily from two species of cyanobacteria: Arthrospira platensis, and Arthrospira maxima. Spirulina can be fed to both saltwater and freshwater fish.

Small Polyp Stony Corals (SPS)
Commonly referred to as Hard Corals. Also, Acropora or Montipora Corals.


This trace element is necessary for corals, clams, and other creatures with calcareous skeletons to grow. It is most commonly added as strontium chloride SrCl2.

A taxonomic category of related organisms ranking between class and an order.

Submerged Plants
Plants that grow fully underwater.

The substrate of an aquarium refers to the material used on the tank bottom. It can affect water chemistry, filtration, and the well-being of the aquarium's inhabitants, and is also an important part of the aquarium's aesthetic appeal.

A low-lying container that holds additional water that flows into/out of the aquarium. Sumps may incorporate or accommodate equipment such as protein skimmers, reactors, heaters or chillers.

Sweeper Tentacles
A coral tentacle or polyp that has an increased number of nematocysts and elongates in order to attack or 'sting' neighbouring corals and sessile invertebrates.

Swim Bladder
An internal air sac which can contain more or less air according to the needs of the fish at the time. More air helps the fish become more buoyant, while less air allows the fish to swim down to deeper levels. Also called Air bladder. Serves to keep fish properly oriented in the water. Also creates and amplifies sounds.

Swim Bladder Disease
A condition in which the Swim Bladder fails to function normally. Causes vary. Symptoms include an inability to maintain normal upright position, or difficulty swimming down to lower levels of the water column.

A close, prolonged association between two or more different organism of different species that may, but does not necessarily, benefit each member (Symbiosis). One of the most famous symbiotic relationships is with a clownfish and anemone.


A Koi that has not reached its full potential a Koi not yet “finished”.

Plural of Taxon. A taxonomic category or group, such as phylum, order, family, genus or species.

The classification of organisms in an ordered system that includes natural relationships.

Trickle Filters
Filters that may include mechanical, chemical and biological attributes and allow water to flow freely from the aquarium and over media.

Total alkalinity
Buffering capacity; is the total amount of all chemical compounds capable of absorbing acids; adequate amount in solution stabilizes pH, allows accurate pH measurements.

Total chlorine
The sum of free chlorine and chloramines (combined chlorine).

Trace elements
A mineral present in water at an extremely low level. Of an interest to the aquarist are those trace elements needed physiologically by fishes or plants; these include cobalt, copper, iodine, iron, magnesium and selenium.
Relating to the visibility within a body of water, more turbid water will have more suspended particulates floating in the water column.

The rate, expressed in the count per hour, at which the total volume of the aquarium water is processed through the filter system back to the aquarium.


Under Gravel Filter
Perforated plates rest on the aquarium bottom and support the gravel substrate. Using lift tubes and power heads, water is pulled down through the gravel, up into the tube, and returned to the water column. UGF provides both mechanical and biological filtration.

Up-Flow System
Water is introduced at the bottom of a filtration unit and then gravity is utilised to allow water to travel up through the media to an exit at the top of the system.

UV (Ultraviolet Light)
Electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. Ultraviolet light is used in aquarium applications in the germicidal wavelength: 185-254 nanometres, which renders unwanted organisms sterile. When organisms can no longer reproduce, they die.

UV Steriliser
Equipment that sterilises water by allowing ultraviolet light to penetrate water through a quartz glass barrier. UV Sterilisers can help remove bacteria, parasites, and algae spores from aquarium water. However, they can also remove some beneficial organisms from reef tanks.


Characterized by or containing vessels that carry or circulate fluids. Such as blood, lymph or sap through the body of an animal or plant.
Ventral Fins     
Pelvic Fins. The most posterior (toward the rear of the fish) set of paired fins on the underside of the fish. Please see "Pelvic Fins".


Water Change
In the marine aquarium hobby, it is the process of replacing a portion of aquarium water with a fresh saltwater mix. This is one way of exporting nutrients from the tank to reduce nitrate and phosphate.

Water Column
The water in the aquarium from the top of the substrate to the surface of the water.

Water Conditioner
A chemical product formulated to treat tap water to make it safe for aquarium use. Water conditioners usually eliminate Chlorine and Chloramines but may also contain ingredients that bind heavy metals, detoxify nitrogen compounds, fortify the Slime Coat of fish, and have other beneficial effects.

Water Hardness
The concentration of calcium and magnesium salts in the water. Also commonly referred to as Alkalinity.
Water Parameters
The readings for measurable factors of water in an aquarium. Basic aquarium water parameters include Ammonium, Nitrite, Nitrate, pH, General Hardness, Salinity and/or Specific Gravity, Temperature and Buffering Capacity.

Water Quality
The chemical balance of the water. Usually referred to when discussing levels of ammonia, nitrite, nitrate and pH in a system.
Wave maker
A device which switches powerheads on and off at timed or random intervals, to simulate wave action in an aquarium.

Wet/Dry Filter
Equipment that facilitates biological filtration by allowing water to flow across a media on which Nitrifying bacteria are attached. The media is not submerged, but kept moist to facilitate the proliferation of aerobic bacteria.

Wild Caught
Wild Caught, Wild Collected or WC - Animals that are taken from native habitat (directly from the wild) and introduced to the captive environment. The captive environment includes wildlife preserves, zoos, conservation facilities, commercial breeding farms, and in the case of tropical fish, the home aquaria. Bringing new species into culture, keeping them alive and propagating them in captivity, can safeguard endangered animals from going extinct. Please see "Captive Bred" and "Captive Environment".


A naturally occurring ore which will absorb ammonia and soften water. It is only effective in fresh water.

Is the branch of biology that focuses on the structure, function, behaviour, and evolution of organisms.

Tiny, free-floating organisms in aquatic systems. Unlike phytoplankton, zooplankton cannot produce their own food, and so consume food from the aquaria such as the corals, rotifers, sea anemones and jellyfish that drift in the water column.  Most are microscopic.

Zooxanthellae Algae
Tiny algae called dinoflagellates that live in a mutually beneficial relation (symbiotically) with corals; they take up waste and carbon dioxide and provide the corals with food (fixed carbon or sugar).