The Nitrogen Cycle
Ardan Huck June 22, 2002

In an aquarium, there are waste products given off by the fish, or by uneaten food or other organics such as plant decay. All contain nitrogen. Fish give off waste products, including ammonia, through the gills. Uneaten food and solid wastes are broken down into ammonia by fungi and many bacteria that we refer to as heterotrophic bacteria.

The ammonia can be in two forms. Toxic (to the fish) ammonia is in the form of NH3 and occurs when the pH is greater than 7.0. Non-toxic ammonia is NH4 and occurs when the pH is below 7.0. Ammonia is very toxic to fish.

Ammonia is broken down into "nitrites" (NO2) by special bacteria called "Nitrosomonas" bacteria.

Then the nitrites are broken down into "nitrates" (NO3) by another special bacterium called "Nitrobacter bacteria." Nitrates are not harmful to fish unless they become high. The most effective way to keep nitrates low is through water changes.

In a new aquarium very little of the nitrosomonas or nitrobacter bacteria are present, and need time to reproduce to sufficient levels to breakdown their respective nitrogen compounds. The bacteria attach to surfaces in the aquarium such as gravel (if you have it) or the material in the filter, such as sponges that have a large surface area for the bacteria to attach to.

Ammonia can start to stress the fish at levels of more than .3 mg/l depending on the pH and the temperature of the water (Untergasser 1989). To minimize the stress, the pH level can be kept below 7.0 and/or water changes can be used to keep the ammonia level below .3 mg/l. Usually the easiest approach is to do water changes. High levels of ammonia may show up on the fish as hemorrhages on the gills. The gills may appear "lilac" in color (Untergasser 1989).

Nitrite in the water is absorbed by the blood of the fish. This reduces the blood's ability to transport oxygen to the fish's body. Continuous exposure to nitrite can lead to "brown blood disease," where the hemoglobin in the blood is bound to nitrite and the fish suffocates from lack of oxygen. Non-iodized salt can be added to the water at 2 tablespoons per 10 gallons of water to help the fish cope with nitrite. Water changes are one of the best ways to cope with high nitrite levels.

When ammonia and nitrite levels reach zero, we refer to the aquarium as being "cycled." This is having enough of the proper bacteria in the aquarium to convert the organics into nitrate. This usually takes from four to six weeks. Speeding up the cycle can be achieved through the addition of gravel or filter media from an already cycled aquarium or by the use of bacteria products sold at a pet shop.

  • Handbook of Fish Diseases by Dieter Untergasser, TFH Publications, Inc 1989
  • Maintaining a Healthy Aquarium by Dr. Neville Carrington, Salamander Books 1985

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