Nitrite Test Kit

Nitrite is produced by beneficial filtration bacteria as they break down the harmful waste ammonia excreted by the fish. Like ammonia, it is not visible to the naked eye. Although it is not as toxic as ammonia, nitrite is still very damaging to the health of the fish. Nitrite itself is broken down into nitrate, which is practically harmless to fish. 
There is no occasion where any nitrite is tolerable in an aquarium. A level of zero mg/L (ppm) should be maintained at all times.