11-17-2008, 12:01 AM
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"Cycling" a tank is the process of culturing colonies of beneficial bacteria in your tank. Fish waste (urine and feces), decaying plant and animal tissue and decaying fish food all create ammonia in your tank. Ammonia is highly toxic to fish and can kill them even at low concentrations. In order for your fish to survive in a fish tank, they can't be exposed to ammonia. Luckily, there is a type of bacteria that converts the harmful ammonia into another chemical called nitrite. As ammonia is introduced to your tank (either by adding fish or another ammonia source) these bacteria multiply. Eventually, there are enough of them to completely convert any ammonia that is introduced to the tank into nitrite. Unfortunately, nitrite is just as toxic to your fish as ammonia, if not moreso. However, there is a second type of bacteria that converts this nitrite into nitrate, a chemical that is only harmful to fish in very large concentrations. As the first type of bacteria convert ammonia to nitrite, the second type of bacteria begins to grow in number. After more time, there are enough of these bacteria present to convert all of your nitrite into nitrate. After both types of bacteria are established, your tank is "cycled." At this point, you should never have detectable levels of ammonia or nitrite in your tank and you only need to do water changes to keep the nitrate levels in check.
There are two ways to cycle a tank, fishless and with fish. When cycling with fish, the fish you add act as the ammonia source during the cycle. However, because the ammonia and nitrite that are produced during the cycle are toxic, you need to do water changes frequently when cycling with fish to keep them alive. The second way is to cycle without fish and use some other ammonia source, such as pure ammonia, fish food or even an uncooked shrimp. This is the preferred method as it allows you to stock the tank as you please (instead of with the fish you cycled with) and also doesn't subject any fish to ammonia or nitrite poisoning.
The best way to monitor the progress of the cycle is to get a good liquid test kit like the API Freshwater Master Test Kit. It contains tests for pH as well as ammonia, nitrite and nitrate. Testing the water lets you know exactly how far along the cycle is and when it's over, and therefore when it's safe to add fish.
Since the bacteria that you grow during the cycle aren't waterborne (they live on surfaces in the aquarium like the gravel, decor and especially the filter media) you can transfer some of these items over to a cycling aquarium from an established tank to help speed up your cycle.
Hope that helped!