Nitrites are an intermediate form of nitrogen produced when ammonia is converted into nitrate during the nitrogen cycle. As organic matter decays, it produces ammonia; the highly toxic ammonia is converted into nitrite by bacteria; and the nitrite is then converted into nitrate by another bacteria.

Nitrite is dangerous to aquatic life if it is allowed to build up, causing tissue damage that can lead to illness and death. By detecting even minute levels of nitrite, the Hanna Nitrite Checker can help aquarists to take action at the earliest possible moment and eliminate unnecessary suffering among the fish.