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Aquarium Nitrogen Cycle

Posted 2/5/2021

ClydeClydeWe have five aquariums in our home. All with various fish that are both entertaining and relaxing. If you are thinking about getting fish as pets, it is vital to do your research on how to properly set up and maintain your aquarium. Whether you want a 5-gallon tank for a betta fish, or a 125-gallon aquarium for a pair of oscars, the set up and maintenance will be very similar. The most important thing to understand about keeping fish, is the nitrogen cycle.

Fish excrete waste as ammonia. Excess food and decaying matter also produce ammonia. Ammonia is highly toxic and ammonia poisoning is the number one reason why new owners lose fish. In the wild, fish can cope due to the large volumes of water they live in, which dilutes the ammonia to non-toxic levels. When we keep fish in a container like a fish tank or pond, it is up to us to create and maintain an environment that is essentially ammonia free. The nitrogen cycle can do this with a little help from us.


Tiger barb tankTiger barb tank


In an established aquarium, bacteria colonies live in the filter media, and small colonies are also present in gravel, on driftwood, or on other surfaces. In the nitrogen cycle, one type of bacteria feeds on the accumulating ammonia and converts it to nitrites; another type of bacteria converts the nitrites to nitrates; and then the nitrates are removed through the absorption by plants or water changes. This completes the cycle. It is important not to disturb the established bacteria colonies by changing the filter media. An occasional rinse in used aquarium water to remove large particles is all that is needed.



A regular part of routine aquarium maintenance is testing for the presence of ammonia, nitrites, and nitrates. When an aquarium is cycled and healthy bacteria are present, there should be minimal to no ammonia or nitrites. You want to see nitrates when testing, as their presence indicates that the nitrogen cycle is performing as it should; but you do not want the levels to get very high, as nitrates can also be toxic at higher levels.  Plants in the aquarium will remove nitrates to a degree, but you need to perform water changes regularly for optimal water quality. (Note: Even if your plants consume enough nitrates to balance your aquarium, please continue to do weekly water changes. Trace elements and minerals from new water are needed to stabilize water quality and are beneficial to both fish and bacteria colonies. In addition, some fish, like goldfish, secrete growth-inhibiting hormones which can interfere with normal development if not diluted with water changes.)


Yellow Peacock CichlidYellow Peacock Cichlid


In a well-maintained aquarium, it is not normal for fish to die often. Unfortunately, fish are often subjected to living in horrible conditions due to uninformed owners. The most common casualty in the fish world is the poor goldfish. Countless goldfish are stuffed into tiny bowls and are slowly (or sometimes quickly) poisoned to death in the increasingly toxic water. It is a myth that goldfish only live for a short time. The average lifespan of a well-tended goldfish is 10-15 years. (The average goldfish will need 20-40 gallons of tank size per fish to be healthy and to reach this age.) An understanding of the nitrogen cycle is the first step towards learning to keep fish and have a successful fish keeping experience.




Just keep swimming!


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