What is nitrite?
Nitrite is produced by the beneficial bacteria in the aquarium and filters as they break down the harmful waste ammonia excreted by the fish. Although 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 virtually harmless to fish.
Why test for nitrite?
Any nitrite present in the aquarium will be very damaging to the health of the fish. Toxic nitrite can build up in aquariums where there are not sufficient beneficial filter bacteria to deal with the total waste from all the fish in the aquarium. This situation can commonly arise in new aquariums, when new fish are added, when the filter bacteria have been disrupted (e.g. during cleaning), or during accidental mis-dosing of aquarium medications.