Tetra Test NO2 to the reliable and exact determination of the nitrite content in fresh and sea water aquaria. Organic nitrogenous substances in the aquarium, such as fish excreta and food remains, are decomposed in stages: first into toxic ammonia, then into nitrite and finally into nitrate (nitrogen cycle). A nitrite content (NO2) of over 0.3 mg/l will cause your fish long-term harm. This test is especially useful when setting up a new aquarium. Packing contents are sufficient for 45 tests.