Cycling the aquarium is the term used to describe the period during which the aquarium matures and reaches a biological equilibrium from its initial set up. It is one of the most critical stages in setting up an aquarium and the most common area of failure for beginners.

In order for filters to process the nitrogenous wastes produced in an aquarium (primarily) by the fish, a colony of bacteria must develop (refer to the section on filters and the Nitrogen Cycle page for more information). The bacteria's job is to break down highly toxic ammonia (and ammonium) to less toxic nitrite and then to convert the nitrite into a safer nitrate compound. It may take over six weeks to develop enough bacteria to cope efficiently with the decomposition of wastes formed in the aquarium, to gain the desired goal of zero ammonia, zero nitrite and the presence of only nitrate. When this is achieved the aquarium has reached its biological equilibrium and can be termed "mature".

A period of around two weeks should be allowed between filling the tank and introducing the first fish. During this time, the minimal number of bacteria naturally present in water must be fed in order to multiply. This can be done in a number of ways. A few handfuls of substrate from a mature disease free aquarium can be added as a bacteria starter culture, a piece of raw meet can be dropped in and allowed to decay or a small pinch of fish food can be added daily. Alternatively, one may use an extremely hardy fish such as a comet goldfish. Your local dealer should be willing to exchange the goldfish once it has served its purpose. This is also an ideal way to "observe" the cycling process as one can monitor the fish's behaviour.

Special additives are also available to speed up the maturation process. Personally I advise against the use of these as the cheaper alternatives outlined above work just as well if not better. The cycling process should not be rushed. Fishkeeping is a hobby which requires patience, it is not a race - one should master this quality from the beginning.

During the first few weeks, ammonia levels will rise rapidly and then fall to zero. Following the presence of ammonia, nitrite will be produced. Nitrite will also peak and fall before the final phase of the cycling process occurs, the presence of nitrate. Be aware though, it is not unknown for nitrite to peak more than once before falling to zero. It is imperative to ensure that the nitrite peak has passed before you add fishes, it may take longer than two weeks so monitor nitrite every other day during the cycling period. There is no need to monitor ammonia levels as these will peak before nitrites, which are the critical factor in establishing whether or not a tank is cycled.

Failure to monitor this process and adding fish too early will result in death of the fish by ammonia and nitrite poisoning. This phenomenon is know as "New Tank Syndrome". The fact that a term exists, indicates that this is an all to common situation.

Once it is established that nitrite has stabilized at zero fish can be added. Do not add too many fish at once and this applies even after the cycling period. I never add more than five fish in any 6 week period even in my largest and oldest tanks. The trick to maintaining a healthy aquarium and avoiding unnecessary deaths is to manage the bioload placed on the filter. That is to ensure the bacteria colony in the filter can cope will the increase of ammonia and nitrite created by every subsequent fish added to the aquarium. It can be a fine balancing act and one which is learned over time but as I mentioned before patience is the key.

When the aquarium is mature and both ammonia and nitrite levels are zero, the remaining concern is the build up of nitrate. Although nitrate is relatively harmless, long term or sudden exposure to high concentrations should be avoided and this is best controlled via partial water changes. As a rule of thumb, 25% to 30% of the aquarium's water should be replaced with fresh water every fortnight. Regular testing for the presence of nitrate should be undertaken to establish when water changes are required to control the rise in nitrate in your particular set up.