Nitrites are also lethal in small amounts, but not as small as ammonia. Nitrites are also broken down by bacteria. This group of bacteria, known as nitrobacters, also requires oxygen to do its work. The nitrites get broken down to nitrates.

Nitrates are not lethal except in extremely large amounts. There are only a few ways to regulate the amount of nitrate. Plants remove nitrate by using it as food. There is a type of bacteria (anaerobic) that will consume nitrate, but oxygen drives them away, and you need a large amount of surface area exposed to very slow moving water in order for them to work. Large amounts of this bacteria are lethal to fish. The other way of removing nitrate is through water changes.

If your tank is new...
The ammonia eating bacteria are not yet established. This means ammonia will build much faster than in an established tank. For this reason alone SMALL fish should be added one or two at a time, and only every two to three weeks for the first four months at the least. Each time a fish is added, the ammonia levels need to be monitored daily. When the ammonia begins to drop, both ammonia and nitrites need to be monitored daily until they drop to zero parts per million (ppm). As time goes on, nitrate levels will continue to increase unless it is removed.

Another way to start your tank is the fishless method. By using pure household ammonia and a test kit, you will be able to track the amonia spikes and not kill any fish. Simply add enough ammonia (a few drops at a time), usually 3 drops per gallon, to read 1 part per million then wait for it to decrease then repeat two more times. Put fish in when the ammonia reads zero.

If your tank is established...
The ammonia spikes will still be there but not as high as in a new tank, as long as fish are added sensibly. This is because the bacteria are already established in the aquarium and only need to increase their numbers in order to compensate for the higher amount of ammonia.