Biological Filtration - Aquarium Nitrogen Cycle
The nitrogen cycle is responsible for the biological fitration in your aquarium. This is important because ammonia from fish waste, uneaten food, and decaying plants all produce ammonia as they break down. Ammonia is very toxic to fish.
An aquarium is a closed system and there is nowhere for these waste products to go unless you do a water change. Thankfully there are bacteria (Nitrosomonas and Nitrobacter) that thrive on ammonia and nitrogenous waste products and help to eliminate these toxins from your aquarium.
These nitrifying bacteria are already present and so you don't need to do anything to get them. However, the reason that it is important to let a new fish tank cycle before adding a bunch of new fish is because in the beginning these bacteria are not present in large enough numbers to break down high amounts of nitrogenous waste. This is why it is beneficial to add just one or two fish in the beginning and then wait 2-3 weeks before adding more. This allows the nitrifying bacteria to grow in number so they can take care of more fish waste. You can also let your new tank cycle by adding a very small amount of fish food each day with no fish present. This will give the nitrifying bacteria something to eat and your tank will be ready for fish in about 2 weeks.
How Biological Filtration Works
Once ammonia is present in the tank, these nitrifying bacteria (the Nitrosomonas) convert it to nitrite. Nitrite is somewhat toxic to fish, but not as toxic to them as ammonia is. However, high levels of nitrite will cause your fish to become ill.
The second group of nitrifying bacteria (Nitrobacter) break down nitrite into nitrate. This is the final product of the nitrification process. Most fish can tolerate nitrate, however, invertebrates may not tolerate high levels of nitrate. Also, even if you are only keeping fish it is best not to let the nitrate levels get too high. Try to keep nitrate levels below 20 ppm.
Live plants use nitrate as a food source and so some nitrate can be eliminated this way.
Another way to remove nitrate (and also some of the ammonia and nitrite that the nitrifying bacteria haven't gotten to yet) is to do regular partial water changes at least once a week. You should change 20 percent of the water each time. Don't change more than this at a time because your fish have gotten used to the water they are living in.
If you perform these partial water changes at least weekly this will keep your ammonia, nitrite, and nitrate levels down, and will keep your fish happy and healthy.
You can check the ammonia, nitrite, and nitrate levels in your tank by using a water test kit. Please see the article Water Test Kits For Aquariums for more information.