about the Nitrogen Cycle



Keeping fish requires an understanding of the environment they live in and the most important part of this is the Nitrgen Cycle.

Following the introduction of fish, plants, and food into your aquarium a process is started which is known as biological filtration or the nitrogen cycle.

This process naturally converts waste materials into less toxic compounds by the use of bacteria.


The first stage of the nitrogen cycle is the formation of ammonia (toxic), or ammonium (non-toxic). In an aquarium more than 50% of the waste produced by fish is in the form of ammonia, the majority of which is secreted through the gills. The remainder of the waste, excreted as fecal matter, undergoes a process called mineralization. Mineralization occurs when Heterotrophic bacteria consume fish waste, decaying plant matter, & uneaten food, converting all 3 to ammonia & other compounds. Ammonia is lethal to fish above certain concentrations & must be removed or broken down.

Efficient filtration is the most effective method of removing ammonia. With ammonia now present in the water the nitrogen cycle is underway. As the ammonia levels rise in the aquarium, a group of bacteria called “Nitrosomonas” feed on the ammonia and convert it to nitrite.


Nitrite is then consumed by another bacteria “Nitrobacter”, and this is converted to nitrates (a relatively nontoxic compound), which is the end product of the biological filtration process.


Nitrate levels will slowly rise in the aquarium, and over a period of time will become toxic to the fish when high levels have accumulated. The filter system will not remove nitrates in the same manner as it removes ammonia and nitrite, and fortnightly partial water changes are required for nitrate control.


The nitrogen cycle can take up to SIX weeks to complete and we advise you follow our stocking levels to keep the toxic lelvels under control.



Urmston Aquatics


To Top