What Is Nitrite? Nitrite is a toxic waste material found in varying concentrations in most aquariums. It is produced by nitrifying bacteria nitrosomonas in the biological filter as it breaks down ammonia. As the biological filter develops and grows, the bacteria nitrobacter use nitrite as a food source, converting it to non-toxic nitrate, thus reducing the level of nitrite in the aquarium. Regular testing for nitrite is important because even low levels of nitrite affect the red blood cells of fish, reducing their ability to carry oxygen, thus causing suffocation and fish death. Testing Tips This test kit reads total nitrite (NO2) levels in parts per million (ppm) from 0 ppm to 5 ppm. Regular testing for nitrite is an essential part of routine aquarium maintenance. Aquarium water should be tested for nitrite every other day when the aquarium is first set up. In established aquariums, test for nitrite weekly to make sure the nitrite level does not rise.