Nitrites

Nitrite (NO2-), which is toxic, is produced through the conversion of ammonia to nitrite. Nitrite can be converted into nitrate (NO3-). 

High levels of nitrites will prevent fish from breathing normally and can quickly lead to fish loss. Even trace amounts suppress fish immune systems and increase likelihood of disease. The presence of nitrite indicates possible over-feeding, too many fish, or inadequate water filtration.  

In an established aquarium, the nitrite level should always remain at 0 ppm; any level above 0 can harm fish. 

(Ref. 2), (Ref. 6)