Fish waste is primarily comprised of Ammonia. The more food you give your fish, the more waste they create. Depending on the pH of your water Ammonia can be quite toxic to your fish if levels become elevated. The presence of Ammonia causes a particular strain of bacteria to develop. This bacteria strain uses the Ammonia and Oxygen as food to transform the Ammonia into Nitrite. Nitrite is also toxic to your fish if levels climb too high. When Nitrite is detected a second strain of bacteria become established. This second strain uses the Nitrite and Oxygen as food to convert the Nitrite into Nitrate. These two strains of bacteria are referred to as Nitrifying bacteria. The exact strain of these bacteria has not yet been positively identified so don't be fooled by the bacteria starter cultures found for sale.

When an aquarium is fully cycled, there are sufficient numbers of these bacteria to process all the Ammonia and Nitrite within your aquarium to Nitrate. Nitrate is relatively harmless unless it is allowed to accumulate upwards approaching 100 ppm (some fish species are more susceptible to Nitrate than others). The easiest way to keep Nitrate levels from climbing this high is by doing frequent partial water changes. While some fish will tolerate higher levels of Nitrate than others, it is recommended that you try to keep them below 20 to 25 ppm. This may be very hard especially if your tap water naturally contains a detectible level of Nitrate as some so. Another factor that will make maintaining Nitrate levels hard is stocking levels or aquarium size. If there are too many fish in a small aquarium of too small an aquarium for the fish kept, Nitrate production will be hard to keep up with. These may sound alike but they are not. Too many fish refers to 20 one inch fish in a ten gallon aquarium and too small an aquarium refers to the recommended size aquarium for the species of fish kept.

In a newly set up aquarium, Nitrifying bacteria colonies are non-existent. Once fish are added the waste they create trigger the process of populating these colonies to begin. Initially, there will not be enough first stage bacteria to handle all the Ammonia present so leftover quantities will be detected. These colonies multiply according to the bio load they have to process. They will continue to increase in numbers until they can successfully process all the Ammonia created by the fish. The same process occurs with the second stage bacteria in converting Nitrite to Nitrate.

If you monitor the Ammonia and Nitrite levels (with a test kit) while this cycling process is progressing, you will see swings in Ammonia levels and Nitrite levels. Initially Ammonia will peak since the first stage bacteria do not have the numbers to handle all that is available. Day by day, these test levels will decrease since first stage bacteria numbers are increasing and leaving less and less free Ammonia in the water. Nitrite levels will do the same but you will not see these same high peaks until at least 3 weeks into the cycling process. When you do see these Nitrite levels peak out to their highest level, this signifies that the first stage bacteria that processes the Ammonia are basically fully populated. At this time, Ammonia should measure 0 ppm (or very close to 0). From now on, Nitrite levels will continue to decrease. Once you measure Ammonia and Nitrites at zero, and they stay there, your cycling has completed. This process will take roughly 4 to 5 weeks.

During this entire process it is important not to disturb what is going on inside your aquarium and filter. This means that water changes, filter maintenance or substrate (gravel) cleaning should NOT be done. Doing any of these will disturb or diminish the Nitrifying bacteria that are trying to populate to the numbers needed to handle the aquariums current bio load. This would result in prolonging the time needed to complete the cycle. There are instances where water changes are necessary for the survival of your fish. If you start to experience fish deaths or dangerously high Ammonia or Nitrite levels, water changes will lower these levels by diluting them.

Now you may begin to add more fish but do not add too many at one time. These bacteria colonies must re-adjust to the new bio load from the increased fish load. This gradual stocking will continue until your aquarium is fully stocked. During the time from initially setting up your aquarium until it is fully stocked, an Ammonia and a Nitrite test kit will be your best friend. These are two test kits you should not be without.

There are way to speed up the cycling process. You can use gravel from an established aquarium, filter material from an established aquarium or an entire filter from an established aquarium. These would already be seeded with both strains of Nitrifying bacteria and allow the bacteria multiplication rate to increase.

If you will have an aquarium with live plants, plant it as heavy as possible with fast growing plants right from the start. By doing this you will most likely never see typical cycling levels of Ammonia or Nitrite. The plants will make use of the Ammonia by stripping out the Nitrogen as one of their necessary nutrients. By doing this, there will be no Ammonia (or very little) leftover to feed the Nitrifying bacteria.

This explanation details what is called conventional cycling. There is also a process called fishless cycling where Ammonia (100%) is added to a fishless aquarium. The benefit of fishless cycling is once cycling is completed a full compliment of fish may be added immediately.