In order to attain a better understanding of the nitrogen cycle, aquarium hobbyists should know more about nitrifying bacteria and the roles they play in the bio-system of an aquarium.

Many different kinds of bacteria are present in our bodies and throughout the environment, including water. There are aerobic (good) bacteria, which require oxygen, and anaerobic (bad) bacteria, which do not require oxygen.

The bacteria in an aquarium will establish themselves on all the surfaces that are suitable for them to live on. These include the gravel, ornaments, tank walls and more importantly, the filter, where a majority of them will accumulate.

Aerobic bacteria need three things to survive: a habitat, food and oxygen. The filter contains a medium that provides a large surface area on which bacteria colonies can establish themselves. The water flowing through the filter provides them with food and oxygen. Food is derived from the decaying organic material in the water column (fish waste, uneaten food, decaying plant matter). And the movement of the flowing water will permit gaseous exchanges to occur, and as a result, oxygen to be produced.

Normally, it takes the good bacteria approximately six to eight weeks to be properly established before they can cope with a new aquarium’s bio-load.

Establishing the nitrogen cycle

The environment in a newly set-up aquarium is a sterile one. At this point, there simply isn’t enough good bacteria to take on the bio-load. Therefore the amount of fish and food added to it should be limited, until adequate bacteria colonies are established.

The first bacteria that develop are the nitrosomonas, which are very hardy and efficient. Once established, these bacteria ingest and break down ammonia into nitrite – this process occurs in the first stage of the nitrogen cycle. Ammonia, a by-product derived from the decomposition of organic matter, is extremely toxic to fish. Its level will peak and then decline, as the growing population of nitrosomonas feed on it.

In the next stage of the cycle, bacteria called nitrobacter will in turn ingest the nitrite and convert it into nitrate, which is the least toxic form of the three by-products. However, unlike nitrosomonas, nitrobacter are slower to develop and less hardy. As they are not as robust and easily affected by chemical treatments and changes in water quality (including temperature), it is common for a nitrite spike (where there’s a high reading in the detection of nitrite in the water) to occur after an aquarium has been fully cycled.

In nature, plants and algae absorb nitrate for use in metabolic processes; but in an aquarium, it is almost impossible to have a large enough number of plants to totally absorb all the nitrate in it. Therefore, the best way to remove nitrate and keep the environment stable is through regular water changes.