Healpful Tips

Tropical Fish Disease

Like all other animals, tropical fish are subject to a number of bacterial, parasitic, fungal and viral infections. Sooner or later, you will come across one or other of these disease problems, so you should be suitably prepared to deal with them.

Fortunately with recent advances in aquarium technology, most of these diseases can be successfully treated, particularly if noticed early, or avoided by providing the fish with a healthy diet and a good water management programme. By doing this the fish will be intrinsically hardier and better immune to diseases.

The two main causes of fish disease, apart from direct exposure to infections, are stress and poor water conditions. Both of these, including the stress of transportation and introduction, are under the control of the aquarist, and both have already been discussed elsewhere in this website.

Disease diagnosis is a vast and complex subject. The following are simplified to allow for recognition and treatment of the most commonly encountered diseases. Please be encouraged to read other references on fish health to learn more about the subject and discover other treatments, in addition to the general health care mentioned here.

Quarantine tank: “Prevention is better than Cure”
A quarantine tank (probably more correctly regarded as an acclimatisation tank) is an essential part of keeping fish in good health. It allows newly purchased specimens to be kept away from the established aquarium so that they can be closely observed for signs of ill-health or parasitic infection which could otherwise spread to the healthy inhabitants of the main aquarium. An adequate acclimatisation tank for small specimens requires 40 litres of suitably heated water with an under gravel filter and decorated lightly with a few rocks for shelter. For larger fish, or when buying numbers of new fish, you will require something proportionately larger.

New fish should be kept in this tank for about two to three weeks, during which time they can be conditioned to the main aquarium’s water and temperature and be treated with suitable medications if required, before their eventual introduction into the main aquarium. By keeping new fish in such a set-up, it is possible to apply medication exclusively to these fish, thus not subjecting the already established inhabitants to the stress, however small, that is inevitable whenever a medicine is added to a healthy tank. Copper treatments in particular (including chelated copper sulphate) are extremely toxic to invertebrates, yet are very effective medications. Keeping new fish in an acclimatisation tank therefore allows the aquarist to make use of these important treatments in complete safety.

Minimising the risks:

It is true to say that the health of any animal is closely related to the environment it is living in. This is even more so with fish, where the environment plays a direct part in the levels of stress felt and their susceptibility to disease. In an aquarium, whenever there is a health problem you need to look at two factors:

  1. The fish themselves: you should maintain a regular routine for observing fish “behaviour” and general appearance at feeding time. This will enable you to immediately locate the ones that are showing any specific symptoms of illness, and to act accordingly to prevent the disease from spreading to other fish in your aquarium.
  2. The environment: you should test and monitor the water quality regularly (read the managing water quality section) so that you can identify and correct all aspects of water chemistry demanded by the fish species in your aquarium at an early stage. In this way, fish or other aquarium inhabitants will not be subjected to any significant levels of stress.
Diseases can and will take place when fish are weak or injured by stress, which may be as a result of poor water quality, incompatible aquarium inhabitants or simply the transportation and handling involved in acquiring new fish. Therefore the first step in combating disease is to prevent it in the first place. By following all the points in our FISH MONITOR checklist you will go a long way to minimising problems within your aquarium.

 F  ollow the correct handling and introduction of fish into your aquarium.
 I  solate any stressed or sick fish for treatment into a quarantine tank.

 S  ervice and clean the system filters regularly & change filter media when necessary.
 H  unt for healthy specimens from a reputable aquatic centre.
 M  aintain appropriate water quality in your system and replenish with trace elements as per dosage on the label.
 O  bserve fish behaviour & appearance carefully at feeding time.
 N  ever overfeed and always remove any uneaten food or detritus build up from the bottom of the tank.
 I  ntroduce only compatible fish into your aquarium or pond.
 T  est your system water regularly and correct all aspects of water chemistry.
 O  rganise yourself by reading some good books on fishkeeping.
 R  egularly make partial water changes to ensure healthy environment.

Common Diseases:

Unfortunately, even when good aquarium maintenance is practiced and fish have been properly quarantined with compatibility in place, there may be rare occasions when there is a disease outbreak. Fortunately, many tropical fish diseases can be cured, particularly if noticed in the early stages. The following Disease Checklist will help you identify the most common tropical fish diseases, including  symptoms, possible cause and treatment.

©Dr. Gerald Bassleer

The disease is caused by the parasite Chilodonella, Chilodonella cyprinid, is widespread and affects all tropical aquarium fish particularly if injured. The infection is caused by a single cell parasite covered in hairs which enable it to swim. The parasite initially attacks injured fish but in crowded aquariums will host on healthy fish slowly debilitating its host. Dead fish must be removed quickly as the cell leaves the body within couple of hours.

Isolate the infected fish into the quarantine tank and use one of the commercially available remedies, in the quarantine tank with clean water and raise the temperature to 28 - 30˚C. One must remember that the parasite may remain free swimming in the infected aquarium, and the whole aquarium must be treated and dealt with to prevent infection. The parasites will eventually die if left in a fish free aquarium for 5 days at 30˚ temperature and ensure that good water quality does exist and the water is well oxygenated.        

©Dr. Gerald Bassleer
This is bacterial infection caused by Flavobaterium columnare bacteria, formerly known as Flexibacter or Chondrococcus. It has been referred to as 'mouth fungus' due to the whitish cotton thread like which appear fungus-like. It is highly contagious and can spread rapidly via contaminated nets. As with many bacterial diseases, poor water quality is the major factor in triggering the disease.

Treatment: Anti-bacterial medications available commercially should cure this disease if used promptly. All the nets should be disinfected. The best cure is prevention by maintaining good water quality.

This is a bacterial infection usually brought about by poor water quality and/or poor diet. Viruses have also been associated with the disease. It damages the heart and blood vessels, the body swells as fluid builds up in the abdomen and behind the eyes. This causes the scales to protrude and sometimes fall away, ulcers may also appear.

Treatment: Commercially available medication can be used successfully if used promptly, it is best to isolate and treat the fish in the quarantine tank and a preventative course of medication is often wise as other fish may be infected but may not yet show signs of the disease. Check the water quality and service the filters and change the filter media.

©Dr. Gerald Bassleer
These Bacterial infections caused by Aeromonas or pseudomonas species which are present in the aquarium water, but do not infect a healthy fish. The disease is often precipitated by poor water quality, low water temperature or where fin damage has occurred by fin-nipping fish. Bacterial infections will weaken the infected fish to the extent that viruses and parasites take hold.

First check the water quality and rectify as appropriate. Then administer the appropriate anti bacterial medication commercially available to destroy the bacteria, while raising the water temperature by a couple of degree will often help halt the damage.               


Fungal spores are always present in the system water. The outbreak of fungal disease usually occurs after damage to the fish skin or gill area, and as a secondary infection, thus only damaged fish are susceptible and healthy fish will be resistant to infection. Usually a wound will attract fungus however; sometime a wound is due to infection by another bacteria or parasite such as worms or flukes. In this case both the fungus and the primary cause of infection must be treated promptly.

Commercially available antifungal remedies can be used to destroy the fungus and prevent it from spreading. Most medication available for fungus will also treat bacterial infections. In a breeding tank, anti-fungal remedies are usually administrated to inhibit fungal growth on the infertile eggs. Good water quality is the key in preventing fungus.   

Gill disease:

The disease can be caused by infections of fungi, bacteria and parasites, however poor water quality is more likely to be the contributory factor.

The first step will be to improve the water quality, avoid over feeding and over crowding, carry out regular water changes to avoid deteriorations. It may be necessary to treat with formalin or copper based medications.  

©Dr. Gerald Bassleer

Hole in the head:

The disease is caused by internal parasites, Hexamita, Sironucleus or Octomitus sp. The outbreaks usually caused by an accumulation of problems which affect the lateral line organ; more likely due to over crowding, poor water quality and poor or unbalanced diet (lack of vegetable and vitamins).

First improve the water quality and give varied diet, then isolate the fish into the quarantine tank and treat with commercially available anti parasite medication, prompt treatment is very important as the holes that parasite creates often leave the fish susceptible to a secondary infection.

Neon disease:

This disease is caused by pleistophora hyphessobryconis parasite, unlike the 'false neon disease' which is bacterial, and shows very similar symptoms. The disease is most likely to be from a newly acquired fish, which have not been properly quarantined; this disease has also been confused with Columnaris disease.

Most commercially available medications are usually ineffective as these parasites are very resistant to medications and the fish will be too weak to be saved by the time diagnosis is confirmed. The best course of action will be to remove the sick fish immediately and human disposal of the infected fish to prevent further infestation.   


Numerous infestations including chronic or acute bacterial infection, metal poisoning and parasitic or fungal infections can cause pop-eye.

Isolate the fish into quarantine tank and depending on the cause, commercially available anti-bacterial and anti-parasite remedies may be effective. Do not feed the fish for three days, sometime, at the end of this period, the eye may be back to normal… often, it will not and you may end up having to destroy the fish humanely.


This is caused by bacterial infection, the bacterium usually occurs as a result of poor water quality.

Improve the water quality by regular partial water changes, this usually will prevent the disease, however, the outbreak of the disease can also be controlled by commercially available anti-bacterial remedies.


The Normal cause may be another disease, look for other symptoms. If this happens to one of the acclimatised fish then it is likely to have been caused by chilling, as a result of which, the fish cannot control its position in the water.

Check your main aquarium temperature; it should not be less than 22˚C, if this is the case, then Isolate the affected fish into the quarantine tank and gradually increase the temperature of the water to 30˚C. The ideal temperature for the aquarium is 25˚C.   


This disease is caused by parasite Oodinium pillularis or Amyloodinium ocellateum; this parasite follows a similar cycle as the white spot (Ich), and may be triggered by the exposure to ammonia and nitrite which is usually associated with poor water quality and the disease is highly infectious.

Improve the water quality by regular water changes and use of commercially available medications containing copper is most effective, however, the main aquarium should also be treated for up to two weeks and increase the water temperature to 28˚C to ensure that the mature parasites have been discharged from the fish and developing cyst are eradicated from the substrate. 

©Dr. Gerald Bassleer
White-spot (Ich):
This disease is caused by parasite Ichthyophthirius multifiliis in freshwater and Cryptocaryon irritans in saltwater. The symptoms usually involve granular, sand-like spots (about 1mm in diameter). Fish are often found scratching themselves against rocks and other hard objects in an attempt to remove the parasite. These parasites are very infectious and have a simple life cycle; however, the highly mobile free swimming larvae must find a host within 48 hours of hatching before they die. Once they find a host, they penetrate the skin and feed on the underlying tissue and body fluids. The white cyst in which they are enclosed gives the characteristic white spot. Once mature, the adults fall from the fish to the bottom of the tank and attach to substrates where they multiply inside the protective capsule after around 18 hours, this capsule burst open, releasing hundreds of free-swimming new larvae which will attach themselves to a new host and the life cycle begins again. White-spot is usually introduced in the aquarium with newly acquired fish that have not been properly quarantined and or exposed to a prolonged cooler than normal temperature.

There are several commercially available remedies, which are based on Malachite green and formalin that are very affective; however, the best method to cure this disease is by attacking the parasite in the free swimming stage where medications become really more affective. Raising the water temperature of the main aquarium to not more that 28˚C, this raise in temperature will help in speeding up the life cycle, so that the free swimming stage is reached much quicker, where it can be treated affectively with medication. Treatments of the whole aquarium should also continue for a further two weeks after all visible signs of the parasite on the fish has gone to make sure that the parasite which may still be present in the substrate in their “gestation” stage are killed too.