Pinned topics on this- absolutely essential reading for any fishkeeper.
Basically, fish excrete ammonia, which is poisonous to them. Cycling means encouraging the growth of friendly bacteria which will turn ammonia into nitrites. Nitrites are also poisonous, but other friendly bacteria will then turn them into nitrates, which are less harmful and can be diluted by regular water changes. There are 2 ways to cycle (=get the bacteria going):
i) fishless, where you add a small dose of pure ammonia to the tank at regular intervals until the test results tell you that the nitrites have gone up and then disappeared- and then you can add fish. This may take a couple of weeks.
ii) with a few hardy fish (such as platies or zebra danios). You need to keep testing the water and be prepared to do water changes if the readings go too high as it can be dangerous for the fish. You need to add fish very slowly if using this method.
