The best way to set up a tank is to stock it heavily with plants, and then add some algae eating shrimp. Take readings for ammonia, nitrite and nitrate every day, and put in a pinch of flake food once a day for a week. Then start adding some fish, starting out with small, peaceful and hardy folks like zebra danios. As the tank matures, the nitrogen cycle will get going. You should get some test strips so you can test every day or two for ammonia, nitrite and nitrate.

The Nitrogen Cycle
This is the single most important thing to understand about how we are able to keep fish in glass cages. A good way to visualize the start-up of the nitrogen cycle is to think of the McDonald's golden "m" arch, but with three arches, each one of which overlaps the next one.

The first arch is ammonia. As the few fish and shrimp in the tank give off waste (ammonia), the arch for ammonia starts going up. When the ammonia arch is near its top, the middle arch will start -- this is nitrite. Over time you will notice that ammonia starts coming down as nitrite goes up. Both ammonia and nitrite are very poisonous to fish and inverts, ammonia being the worst, and especially if the pH is above 7.0. Soon the middle arch (nitrite) peaks, and the third arch starts on its way up -- this is nitrate. Nitrate is not that harmful to fish, and it can be removed from the water by having lots of fast growing plants in the tank, and doing a regular water change of 25 percent once a week.

When a tank has "cycled" that means that the various bacteria that work the magic of the nitrogen cycle -- that change ammonia into nitrite, and then nitrite into nitrate -- have grown to the proper quantities needed for the tank. When the tank has cycled you can safely add some more fish -- a few at a time. A mature tank will be able to keep the nitrogen cycle going since the good bacteria will increase in numbers as you add more fish.