You may have heard that your tank needs to "Cycle". No, you don't have to put your new aquarium on a stationary bike. This is the Nitrogen Cycle. It's the reason we can't just add all the fish we want immediately after getting our new tank. Some of you probably found out the hard way and lost most, if not all, of those new fish. Commonly known as "New Tank Syndrome" for those of us who have a hard time with patience (erm....me).
Basically, the main cause of all these problems is...fish poop. Yuk! Fish poop causes a sharp rise in ammonia, which is very toxic to the fish. Normally, bacteria present in the substrate and live rock break-down the ammonia to nitrite and then to nitrate, which is much less toxic to your fish. In a new tank, the beneficial bacteria haven't had a chance to populate yet, and they can't keep up with the fish poop produced by the new fish. So the fish die, and you get, whallah! New Tank Syndrome. So you need bacteria. This is one of the few cases where a clean tank is an unhappy tank.
The key is patience. It takes 4-6 weeks for your new tank to fully cycle. In the meantime, use your test kits to track the changes in ammonia, nitrite, and nitrate levels. When the ammonia level drops to 0 and the nitrate levels begin to rise, it's time for your first water change (about 50%). You'll know when your tank has fully "cycled" when the ammonia levels drop to 0. You're then just about ready to add fish. Be sure to add only a few (1-3). The addition of each fish will actually trigger a 2nd "mini cycle" in your tank. Your beneficial, nitrifying bacteria population is still growing, and the addition of too many fish can easily overhwhelm the entire system. Add a few at a time, allow the good bacteria to grow for another month, and then add a few more.
If you have live rock, you're in luck. You have a well-established seeding of nitrifying bacteria ready to go. All you have to do is wait. You may never see any ammonia or nitrite levels. You will know when it's ready when the nitrate levels rise a bit after a month or so. Yes, it still takes the same amount of time. However, down the road, your biological filter (live rock) will be enormous and able to support the bioload produced by your fish.
You may have heard the best way to begin the cycle is to introduce a hardy fish, such as a damsel. However, this is not a great idea. First, it's very cruel to the fish. Second, damsels, which are most commonly used for this, become quite the aggressive pest to your other prized fish down the road.
Also avoid additives that claim to remove ammonia from the water or boost the cycling process overnight. These products are highly unreliable and can actually interfere with the cycling process. Be patient, and you will be rewarded. You will have much happier fish in the end!
Nitrogen Cycling in the Aquarium Links: