Whether you are having a saltwater or freshwater tank, or having a fish only tank or reef tank, the nitrogen cycle is a must know to help you establish a successful tank.

The aquarium is a sealed environment. All waste from the fish, uneaten food and rotten algae or plants will remain in the tank and break down into some poisonous sustains. In order to remove these poisonous substances, we need different kinds of bacteria. These bacteria gradually grow in the tank and began to consume those poisonous substances into some safer by-products. To keep our fish safe and happy, we need to understand what’s actually happening inside our tank so that we know how to improve our tank and why our fish dies.
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There are 3 stages of the nitrogen cycle, and each with a different function.
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The First Stage
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When you introduce the first fish into the tank, the nitrogen cycle begins. Waste , urine, feces, and any leftover food will break down into ammonia(NH3) which is poisonous to fish and corals. It is fatal to fish when the ammonia concentration reaches 2ppm and a mature aquarium should always have a 0ppm of ammonia concentration. In order to keep track of the ammonia level in your tank, tester should be used.

​The Second Stage

​At this stage, oxygen-loving bacteria called nitrosomonas will develop and they will oxidize the ammonia to become nitrite(NO2). Nitrite is toxic to fish, but less than ammonia. In order to end this stage faster, filter media such as bio rings and live rocks fit this job very well because they provide large surface for the nitrosomonas to settle down and do their job to eliminate ammonia. Also, the nitrosomonas need a large amount of oxygen to oxidize ammonia, a stong water current should be intoduced. So the filter media should be placed inside a strong filter like a canister filter.
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​The Third Stage

In the final stage of the nitrogen cycle, bacteria called nitrobacter will consume nitrite as a food and produce nitrate (NO3). Comparing to nitrite and ammonia, nitrate is not fatal to fish in the low to moderate concentration. However, a high concentration of nitrate is still fatal to fish. There are a few ways to reduce the concentration of nitrate, the most effective and simplest way is to do a partial water change. Anaerobic bacteria can be used to eliminate nitrate to nitrogen through the process called denitrification.