The final stage of the Nitrogen Cycle in the aquarium (apart from those aquaria with an established nitrate removal system) is the breakdown of nitrite by nitrobacter bacteria to nitrate.

Nitrate is certainly less toxic to fish than either ammonia or nitrite and until recently considered to be harmless unless in very high concentrations. Studies have since shown that although nitrate may not be lethal unless in exceptionally high quantities it does make fish more susceptible to disease and reduces the likelihood of breeding success.

The other problem with high nitrates is the possibility of inducing nitrate shock to new fish when introducing them to a tank containing higher nitrates than they are accustomed to. As with nitrite, some fish are more sensitive to the effects of nitrate than others.

A level of 0 mg/l (zero parts per million) in a freshwater tank is difficult to achieve however one must try to maintain as low a nitrate level as possible. Certainly levels above 20 mg/l should be avoided for sensitive fish such as Discus and levels below 40 mg/l should acceptable for most other fish. Check nitrates regularly with a test kit.