Taking a few minutes each week to maintain and monitor aquarium conditions is the key to a healthy aquarium. Nitrite is produced in the aquarium by fish waste and uneaten fish food. Toxic nitrite is produced by nitrifying bacteria in the biological filter as it breaks down ammonia. Low levels of nitrite inhibit respiration and suppress the immune system; high levels cause suffocation. The Nitrite Test Kit tests for harmful nitrite levels from 0 to 5 ppm.