how to start the nitrogen cycle in a fish tank

Understanding the nitrogen cycle and its role in the aquarium is vital in order to maintain a healthy environment for your fish.

Once fish are introduced to a body of water inevitably the initial quality of water will degrade. Fish excrete, food remains rot and plant leaves decay. The stages by which these waste products are broken down by bacteria is termed the nitrogen cycle.

nitrogen cycle aquarium


Rotting debris and excreta from the fish produce highly toxic ammonia (or in acid waters ammonium), which pollutes the water.


Nitrosomonas bacteria convert the toxic ammonia (or ammonium) into less toxic, but still dangerously harmful nitrite (NO2).

Nitrite is toxic to fish at levels of above 1 mg per litre. Even in a mature aquarium there will still be a trace (less than 1 mg/ltr or parts per million) of ammonia (or ammonium) present as the conversion to nitrite is not an instantaneous one. But as with ammonia, the ideal level of nitrite should be zero.


Nitrobacter act on the nitrite to convert it to less poisonous nitrate (NO3).

Although nitrate is relatively harmless, long term or sudden exposure to high concentrations should be avoided. Depending on the species, fish are susceptible to nitrate levels above 100 mg per litre.

Above levels of 150 mg per litre denitrification occurs, that is the the reduction of nitrate to nitrite to ammonia increases. One should aim to keep nitrate levels below 40 mg per litre and less than 20 mg per litre for sensitive fish.


Plants use nitrate as a fertilizer. An abundance of plants helps to control the build up of nitrates however, as the fish population density in an aquarium is higher than in a comparable body of water in the wild, i.e. the system is unbalanced.

One would require an aquatic jungle alongside few fish to make a real impact on nitrate reduction. The best way to control the build up of nitrate is via regular water changes.


Fish and other animals eat plants and so the cycle begins again.

The nitrogen cycle cannot occur without the presence of oxygen. If the supply of oxygen is low, organic matter decays more slowly and the water becomes abundant with the toxic interim products – ammonia and nitrite.

It should also be noted that the Nitrosomonas and Nitrobacter bacteria are not immediately present in a new aquarium and must multiply and develop over time. It may up to 6 weeks for enough bacteria to colonize before the nitrogen cycle can be accomplished efficiently. This process is known as cycling the tank.