Nitrogen cycling is the initial and essential bacterial establishment of a new tank. The tank must cycle in order for your fish to have a supported ecological system for them to live. Nitrogen is a compound formed by ammonia and nitrites which comes from uneaten food and fish excretions. Nitrate is a compound commonly used up as fertilizer for plants and other needy organisms like clams. Without nitrate, many inverts and plants would find it impossible to survive, yet when nitrate is present in large quantities, it can have an opposite and deadly effect. Tank cycling is an essential process in which vital bacterial populations grow throughout provided surface area in the tank in order to break down fish products into less harmful substances. When starting the cycle you may need to wait up to 1 1/2 or 2 months before you can put sensitive fish for example butterflyfish. A good way to get the cycle started is to get a few damsels (hardy/tolerable fish) and wait for a couple months, if you choose not to start with damsels then you should find some other source of ammonia like a dead fish or some fish food and let it sit a while. In addition, you may want to add some beneficial bacteria to decrease the amount of time you'll need to wait to get the tank cycled. Live rock is another instant way to help your biological filtration out and your tank cycle process. The rock consists of all the essential bacteria as well as tiny organisms that feed on harmful ammonia and nitrite in the water. This provides for a healthier and more natural looking tank for your fish. When purchasing live rock, make sure they are cured (i.e. do not smell rotten and have been sitting long enough for no decay or ammonia traces). When adding live rock to the tank there is always a chance that you could have picked uncured live rock and the ammonia levels will be at harmful levels due to the decaying matter on the rocks. Many weak organisms on the rock die in route to their retailers leaving behind traces of ammonia which needs time to heal itself. The most toxic substance for fish when cycling is ammonia, but as your tank cycles, ammonia breaks down into nitrites and then to nitrates, which is less toxic to your fish. Basically in a nut shell, what you are waiting for during the 1 1/2 month period is for special bacteria populations to sufficiently become stable enough in the tank in order to biologically handle feeding on fish waste. In order for the bacteria to grow it needs ammonia as a food source which is what the waste from the damselfish provide for the first few weeks. Once these bacterial populations have established, any ammonia entering the tank will be consumed rapidly enough for fish to live unharmed by it. Knowing this, it is easy for one to understand the importance of live rock in that you would be introducing all the essential bacteria to the tank. The main thing you want to do when cycling a tank is to have patience and not put too many fish in without cycling first. You can lose lots of money and fish by not cycling your tank. Down below shows the full cycling process.