Nitrite is the result of the bacterial conversion of ammonia. The reason for detectable nitrite is linked to high ammonia levels (see high ammonia). The readings should be 0 at all times.
Nitrite damages the nervous system, liver, spleen, and kidneys of the fish. Even lower concentrations over extended periods can cause long term damage. Nitrite binds the oxygen carrying hemoglobin in blood; fish can suffocate even if the oxygen in the tank is sufficient.
Nitrite poisoning can be observed, if the fish is grasping for air at the surface.
