Aquatics Staff, Drs. Foster & Smith, Inc.
Understanding the nitrogen cycle is an important part of keeping a successful aquarium. The nitrogen cycle is responsible for the biological filtration within the system. It keeps the water free of toxic compounds that are a result of the respiration of the inhabitants, and the decay of any matter such as waste products and uneaten food. When we understand this cycle, we can anticipate situations that may cause damage to this process, and prevent or avoid these situations that may lead to livestock loss.
What is the nitrogen cycle?
In the nitrogen cycle, the waste products of the fish, plants, and invertebrates, along with any dead organisms or uneaten food, are broken down by bacteria and fungi into the resulting chemical, ammonia. Ammonia is extremely toxic to all of the aquarium inhabitants. It is broken down by an oxygen-loving bacteria, Nitrosomonas. The Nitrosomonas* bacteria feed on both oxygen and ammonia, and with their biological activities, they excrete a chemical called nitrite. Although nitrite is not as toxic as ammonia, even at low concentrations in the aquarium, it can be harmful to fish and invertebrates. Another bacteria Nitrobacter*, which also utilizes oxygen in its respiration, acts in a similar way as Nitrosomonas, and essentially changes the nitrites into a relatively harmless chemical called nitrate. The bacteria that will feed on nitrates are anaerobic, meaning they grow in areas of little or no oxygen. They require low-oxygenated stagnant water, and can be found in more elaborate filtration systems and within live rock. Here they breakdown nitrates into free nitrogen.
* Note: Recent research by one of the leading companies in the aquatics industry has found that other bacteria (some of which are still unnamed) are involved in the nitrogen cycle. The company has isolated these strains of bacteria in the freshwater systems and will be marketing them in a product to be used as a cycling aid. A product containing the saltwater strains is still in development, but is expected to be available in the near future. In this article, I have used the names of the bacteria commonly referred to in current textbooks and journals, understanding these names may be added to or changed as we learn more. For home aquarists, the names of the specific bacteria are not as important as understanding the process and what may affect its efficiency.
The nitrogen cycle in new aquariums
Newly-set-up aquariums lack the colonies of bacteria that are necessary to perform the biological filtration. Because of this, the aquarium must be "cycled." "Cycling" refers to the process of establishing and maturing the biological filtration. In order to establish the system, we need to provide a source of ammonia for the Nitrosomonas bacteria in the filtration system so they can live, reproduce, and colonize. To provide an ammonia source, it is best to add a few hardy fish that can withstand the presence of ammonia and nitrites. Then we need to seed the aquarium with bacteria. There are commercially available cycling aids that contain the bacteria. Otherwise, when purchasing the hardy fish, request a small amount of gravel from the aquarium where the fish were held. This gravel should then be placed in the new aquarium along with the fish, and will provide the bacteria that are necessary to seed the system.
As the fish in the new system are fed and begin to thrive, they will, through their biological activities, produce ammonia. The Nitrosomonas bacteria, in turn, will begin to feed upon that ammonia and will start populating the aquarium. Their population will be greatest in the media that contains the highest level of oxygen and surface area, which will normally be within the filtration system. At this point, because the numbers of bacteria are limited, they will not be able to convert all of the ammonia that is present in the system, so the ammonia levels will continue to rise. As the amount of ammonia increases, the population of bacteria will also increase, but at a much slower rate than the ammonia. The ammonia level will eventually reach a peak and then start to decline as the population of bacteria becomes large enough to break down the ammonia faster than it is being produced. Because there is still ammonia within the system, however, the bacteria will continue to live and feed on the ammonia until it reaches a level undetectable by testing. At this point, a balance has been achieved in which the rate of ammonia production equals the rate at which it is broken down by the bacteria. The number of bacteria, from this point on, will change as the levels of ammonia (their food source) changes.
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