healthy Nitrogen Cycle is a must for any aquarium to support fish successfully. One of the best indicators of the Nitrogen Cycle’s health is that of nitrite (NO₂^1-) and nitrate (NO₃^1-) concentrations. The Nitrogen Cycle is the bacterial conversion of ammonia (NH₃) to nitrite (NO₂^1-), and eventually to nitrate (NO₃^1-). In a newly setup aquarium, the biological filter (where the Nitrogen Cycle occurs) hasn’t had time to become established, so ammonia levels elevate quickly. Eventually ammonia levels fall to zero as it’s converted to nitrite. As ammonia levels fall, nitrite levels begin to rise sharply. Eventually, nitrite levels fall to zero as it’s converted to nitrate. From this point on, nitrate levels will slowly rise as the nitrogen cycle proceeds -- unless the nitrate is removed somehow. This whole process is commonly known as "The Break-In Cycle" or "New Tank Syndrome". Even in an established aquarium, ammonia and/or nitrite levels can spike (say from the external biological filter becoming clogged, or from the death and decomposition of a large plant or fish). So regularly monitoring ammonia and nitrite levels is a must, and immediate remedy of any spikes is equally as important.