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The Biological Cycle: A simplified explanation

© Winston Vaughan Schoenfeld

The biological cycle of a new saltwater aquarium is something which should be understood before starting a new aquarium. Many new aquarists start new setups without knowing anything about what is going on biologically. While you can get by without an understanding of the biological cycle, understanding it will help an aquarist to prevent many of the common problems that they may encounter. In this article, I will try to explain the biological cycle of aquariums in an easy to understand format.

There are three major components in the aquarium which one must consider: ammonia (NH3), nitrite (NO2), and nitrate (NO3). Of these, only the first two are toxic to the majority of saltwater inhabitants (actually, the last one, nitrate, is toxic to some saltwater plants, but you won't have to worry about it in a home aquarium). In a new aqurium, none of these is present in any concentrated amount. The cycle of an aquarium begins with ammonia. There are two main sources of ammonia in an aquarium: that which the animals produce from breaking down the protein in the food, and from any bacterial decay which may occur due to the death of an animal. Ammonia levels of only a few parts per million (ppm) can easily kill fish and other animals in an aquarium.

Luckily, there are nitrifying bacteria which break down the ammonia into nitrite. These bacteria are referred to as Nitrosomonas, Nitrosopira, Nitrosococens, and Nitrosolobus (don't worry too much about these particular names). The nitrite which is now in the tank due to the previous bacteria, is also toxic to aquarium inhabitants, but there are additional bacteria in the tank which then break down the nitrite into the relatively non-toxic nitrate (it is about 1000 times less toxic than nitrite). These bacteria are known as Nitrobacter, Nitrospira, Nitrocystis , and Nitrococcus. At this point, the nitrate continues to accumulate at the bottom of the tank, until a water change is performed to remove it.

The entire process described above relies on the presence of bacteria to break down the toxic components which could otherwise reach deadly levels in the aquarium. This becomes an important concept when considering the biological process of a new aquarium. When a new aquarium is set up, it doesn't have the necessary nitrifying bacteria to regulate the levels of the ammonia in the tank. As a result, the aquarium's ammonia levels will initially increase to high levels and reach a peak about nine days after the aquarium was first set up. At this point, the nitrifying bacteria which convert ammonia to nitrite have begun to form, and within six

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