- Category: New Aquarium Setup
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Probably the most important step in starting a new aquarium is letting the tank cycle. Cycling is the process that starts the biological filter. During the cycle, the necessary colonies of Nitrosomonas and Nitrospira each take hold and grow. The colonization of the filter and gravel by Nitrosomonas (AOB) initiates the nitrogen cycle. After this, a natural succession occurs where the AOB convert ammonia in the aquarium to nitrite. Before the next wave of bacteria, Nitrobacter and/or Nitrospira, can colonize the aquarium two conditions must be met. First, there must be sufficient nitrite in the water for these NOB to take hold. Second, since growth of these bacteria is somewhat retarded by ammonia, the ammonia concentration must be brought nearly to zero by the AOB. So there is a population dynamic where the AOB population explodes because of the large amounts of ammonia in the water. They quickly use this up and produce a nitrite spike. Since the ammonia level is low, the AOB population starts to die back. At the same time, the nitrite spike fuels an explosion of NOB. When the nitrite is used up, the NOB start to die back. All of these dying bacteria, together with whatever food is being added to the system, decomposes to ammonia, which feeds the AOB population. The pendulum swings back and forth until the AOB population just keeps up with ammonia input, and the NOB population just keeps up with nitrite production. When these cultures are in equilibrium, the ammonia and nitrite concentrations are maintained at or near zero and the tank is said to be cycled.
This diagram shows the progression of ammonia, nitrite, and nitrate levels during cycling. The values were taken from my African cichlid tank when I first set it up. I colored the curves to illustrate the toxicity of the various compounds. While there is a small green "safe" zone marked for ammonia and nitrite, these should be maintained at zero in the aquarium. I'm sure there is a "safe" level of carbon monoxide, but I wouldn't buy a house that had it!
The procedure for cycling a tank is simple. Just fill the tank with water, turn on the filter, and wait. Establishing the necessary bacterial cultures is not a rapid process and can take four to six weeks. Be patient! The only things you should do to hurry the cycle along is add small amounts of food, or add some gravel or filter media from a healthy, established aquarium (or both). Flake food or tiny bits of shrimp work well. These small food inputs will provide ammonia and eventually nitrite to feed the cultures. Many tropical fish retailers sell cultures of nitrifying bacteria to make the tank cycle more quickly by establishing the bacterial populations faster. The number of living cells in these preparations varies greatly, and I have not read of anyone strongly recommending them. Many older aquarium books and most fish retailers tell people to cycle the tank using one or two hardy fish. This is not only unnecessary, it's cruel. The water in a brand new aquarium is nowhere near adequate for supporting aquatic life! Test the water for ammonia and nitrite once a week for the first two weeks, then every few days after that. Once the levels reach zero, it is safe to begin stocking the aquarium - slowly. Add fish one or two at a time if they are large, 3-6 is O.K. if they are small. If too many fish are added too quickly the waste they produce can overwhelm the biological filter and cause an ammonia spike. Remember that any sort of medication added to the main tank will kill your biological filter as well as the disease you are treating. This effectively "uncycles" the tank. If you medicate you main tank (I advise against it - get a cheap hospital tank) you need to pay careful attention to ammonia and nitrite levels and be ready to do daily water changes.