The Nitrogen Cycle

The Nitrogen Cycle
by Marc Elieson
In this article, I will discuss the Nitrogen Cycle and how to cycle a new aquarium.

Cycling an aquarium refers to a process that takes place in the first 3-4 weeks of a new tank�s setup. If this process is understood and respected, your aquatic experience can become an enjoyable one. If not, your experience may be very frustrating. I am convinced that most new aquarists quit the hobby because they do not understand and respect this process once they�ve set up their new tank.

So what is this process?

During the first four to six weeks after you have set up a new tank, it is busy trying to reach a biological equilibrium. This entails, for the most part, the establishment and cultivation of a healthy bacteria colony in your tank�s filters and gravel. This is important because without such bacteria, your tank has no way to cope with the toxic ammonia produced by your fish and dead plant matter. It�s the bacteria�s job to break down the highly toxic ammonia to a slightly less toxic form called nitrite. Other bacteria then break nitrite down into a relatively more safe compound called nitrate. These two colonies of bacteria, nitrosomonas and nitrobacter, are known as “nitrifying and denitrifying bacteria” because they break down the nitrogen in ammonia to a form that can then be utilized by plants and algae.

Nitrogen Cycle
It takes about two weeks for the first group of bacteria (nitrosomonas) to become mature enough to convert all the ammonia in your tank to nitrite. During this period, your ammonia levels will steadily increase, potentially reaching very toxic levels. Following the establishment of nitrosomonas bacteria, it takes the next group of bacteria (nitrobacter) about 14 days to mature to a level that they can convert all of your tank�s nitrite into nitrate.

Once your tank has an established colony of nitrifying and denitrifying bacteria, your ammonia and nitrite levels should be zero, or very close to it. Your nitrate levels, on the other hand, will continue to increase over time; that is, unless you have a means of removing it. While nitrate is much more safe than its predecessors, it can become toxic if you let it build up ad finitum. The two most common methods for removing nitrate are either through the use of aquarium plants (who need it to grow) or through regular water changes. Note in the diagram below that a single water change will not permanently solve your problem; it will just alleviate the problem for a week or two. For this reason, regular water changes are strongly recommended.

Nitrogen Cycle
As a rule of thumb, 25 � 30% of the aquarium�s water should be replaced with fresh water every two weeks. Alternatively, you could half this figure and perform 10 – 15% water changes once a week. Regular testing of your water�s nitrate levels are advised so that you will be alerted to any sudden increases or toxic levels. This practice will help you to know when water changes are necessary.

To begin cycling your tank once you�ve set it up you need to supply it with an ammonia source. There are a couple of ways this can be done. The simplest and cheapest way to do this is to purchase one or two really inexpensive or hardy fish. Alternatively, you could purchase pure ammonia from the grocery store and cycle your tank without fish. This second method is referred to as “fishless cycling.” If you prefer to go with some hardy fish, Black Skirt Tetras can survive just about anything. Once these fish have cycled your tank, you should be able to trade them back to your LFS.

You want to be careful not to overfeed these fish because the more you feed them the more feces they�ll produce, and more feces means more ammonia. If the ammonia gets high too quickly they will probably die before your tank is done cycling. Also, it is very important that you not put more than one or two fish in your tank to cycle it. Ammonia doesn�t just come from uneaten food and fecal matter, but a surprising amount also comes from the fish�s gills as a byproduct of respiration.

Note: Do not use feeder fish to cycle your tank because these often harbor parasites and may be extremely unhealthy. Using them for such a purpose may introduce unwanted diseases into your tank.

You should wait a period of at least two weeks before you add any additional fish, and especially any fish you want to survive. The cycling process cannot and should not be rushed. Fish require lots of patience. It�s best you learn this virtue from the start. If you do decide to rush this step and introduce more fish, they will succumb to death due to ammonia and nitrite poisoning. This phenomenon is commonly called “New Tank Syndrome.” I will reiterate that it is my opinion that most people who quit the hobby do so because they rush this step. They can�t wait to get their tank all set up and stocked. But to their dismay, the fish only survive two or three weeks. They become frustrated at losing the fish and their money. In the end, they give up the hobby because of ignorance or because they were unable to wait until their tank finished cycling.

It is critical that your nitrite levels stabilize at zero before you introduce more fish into the tank. This may take more than a couple of weeks, so monitor your nitrite levels every other day during the cycling period. You don�t need to bother monitoring the ammonia because it will peak before the nitrite, which is the critical factor in determining if your tank has completed its cycling period.

The cycling process is a rite of passage, if you will. Endure these four weeks of patience and good fish husbandry and you’ll be initiated into the inner circle of enjoyable fish keeping for years to come. I strongly encourage you to respect and understand the nitrogen cycle so that your experience with cichlids will be as hassle free as possible.


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