The Nitrogen Cycle

The nitrogen cycle is important to understand for the safety of your fish. Fish waste and excessive feeding contribute to the creation of compounds that are dangerous to fish. Bacteria break down waste and it transitions through several different compounds including. It ultimately ends up as the least dangerous compound which is removed from the water through water changes.

By Ilmari Karonen [Public domain]
By Ilmari Karonen [Public domain]


Fish waste and excessive food fall to the ground of the aquarium and breaks down into ammonia. Ammonia is very dangerous to fish and in large doses is almost always deadly. In small doses, fish can tolerate ammonia, but as long as there are fish in your tank, this there is a source of ammonia adding more and more every day. Fortunately, a bacteria called nitrosomonas which consume ammonia, will begin to establish itself in the aquarium.  The nitrosomonas will find its sweet spot in terms of population where it will quickly consume all the ammonia produces while appropriately feeding all of the bacteria.  The waste produced by nitrosomonas is another compound called nitrites.


Nitrites are slightly less toxic to fish than ammonia.  However, at any level, they still cause stress and discomfort.  And much like ammonia, nitrites are deadly to fish at high levels.  As with ammonia, there is a bacteria, called nitrobacter, that consumes nitrites.  If you have a live plants in your tank, they will also consume some nitrites.  Nitrobacter excrete nitrates, taking us to the last stage of the aquarium nitrogen cycle.


Fish can tolerate much higher levels of nitrates than ammonia and nitrites. Whereas any level of ammonia or nitrites are considered toxic to fish, nitrates can reach approximately 40 parts per million (ppm) before fish experience any adverse effects.  Excessive nitrates are removed from your tank by doing water changes.  Also, if you have live plants, they will consume some of the nitrates produced.