Cycling is basically a process that establishes beneficial bacteria in your aquarium. This beneficial bacteria eats harmful toxins in your tank.
Basically, the wastes your fish leave in the tank (left over food, poop and pee! Yes fish pee!) become ammonia (NH3,NH4) in the water. The beneficial bacteria (which usually lives in the filter, or attach themselves to the gravel... which is why people usually tell you not to clean your rocks unless absolutely necessary! But a filter is probably more beneficial.) breaks it down into Nitrite(NO2), which is in fact more dangerous than ammonia. Then another type of bacteria will break the Nitrite into Nitrate(NO3) which is generally harmless. Although high concentrations of Nitrate will stress your fish out, so that's why water changes are essential.
There are several ways to cycle a tank, but I usually cheat and just use Nutrafin Cycle, which is like beneficial bacteria in a bottle... add some straight to my filter after a water change or after setting up a new tank, let it sit with the filter running for a bit, test and then put the fish into the water :)
If you don't cycle your tank, you'll have to do water changes much more frequently. Although I still do water changes every week to be safe :)
Last edited by BakaMandy; 09-09-2009 at 05:30 PM.