Healpful Tips

Nitrate & Algae Control

Nitrate is the end product of the nitrification bacteria as they break down the toxic ammonia and nitrite. Nitrate is not toxic to the aquarium or pond inhabitant if kept at low levels. Nitrate is a plant nutrient and any increase will be ideal conditions for undesirable algae growth. Algae need light and nutrients such as nitrate to grow. In order to ultimately control the algae the nitrate level must be limited by the following techniques:

1.  Limiting the source of the nitrate by limiting the amount of fish and feeding, as we know from the nitrogen cycle, fish waste are rich in ammonia and are eventually broken down by the nitrification bacteria and convert into nitrate, so by limiting the amount of waste product the nitrate will be limited too. Fish food also adds phosphate into the water, which will further encourage the algae growth; therefore, it is very important to remove any uneaten food.

2.  There is a phrase frequently used amongst environmentalists and ecologists, that “the solution to pollution is dilution”, this is very relevant to aquarists. Over time nitrates will build up due to constant release of waste products and performing regular water changes of 10 – 15 % will dilute the nitrate thus lowering the concentration of the nitrate, thereby limiting the amount available to algae.

3.  Denitrification, experience and knowledge has been exploited to produce “anaerobic filters” or oxygen free filters, through which the nitrate rich water is pumped very slowly over a dense packing of carbon media, resulting in oxygen free environment, thus increasing the biomass of denitrification bacteria. In other words, the bacteria that produce nitrite NO2 and nitrate NO3 are aerobic bacteria; they will add an oxygen atom to nitrite and convert into nitrate. However if these bacteria, and the many environmental bacteria alike, should find themselves in an anaerobic environment (zero oxygen concentration), then they use nitrate and nitrite as a source of oxygen, breaking NO3 and NO2 down into oxygen and free nitrogen as shown in the denitrification diagram. 


Nitrate is oxidised to nitrite, then on to ammonia, or free nitrogen. (If ammonia is produced then the process is not strictly denitrification). Each stage of this process gives off an oxygen atom. Thus nitrate is the source of oxygen free (anaerobic) environment.

Therefore, it is recommended not to use one technique at the expense of another. The best policy should involve using all the above techniques. Finally remember that a small amount of algae is natural and a sign of a healthy environment and even beneficial to the herbivore species. The key factor is preventing the dominance of algae by limiting the available nutrients.