First of all, let's get an understanding of what "cycling" an aquarium actually is. More correctly described as the nitrification cycle, the process consists of growing certain types of beneficial bacteria on the surfaces inside your tank and filters. These bacteria consume waste products produced by the aquarium inhabitants which is primarily ammonia. Other waste products, such as feces, excess food and plant debris should be removed regularly, but if left in the tank, they will break down and give off ammonia as well. The type of bacteria that oxidize ammonia as part of their metabolic process are called Nitrosomonas. They consume the ammonia and change it into nitrite. When nitrite appears in the tank another bacteria, called Nitrobacter starts to grow and these oxidize nitrite into nitrate. Both of these types of bacteria will appear in the tank eventually if ammonia is present - it is impossible to know where they come from - the air, the tap water, the dust and microbes on the equipment. They just appear and form colonies, but there are ways to help them along which I will address later in this article. Both ammonia and nitrite are toxic to fish. Nitrate is not harmful to fish until it reaches high levels. (Newcomers to the world of testing sometimes confuse the words 'nitrite' and 'nitrate' because the spelling is so close. Pay attention to the vowels to understand properly!) So, when a fish tank "cycles" it is going through the process of growing bacterial colonies to consume toxic waste products in the aquarium. The process looks something like this: Ammonia is added to water every day ---- Nitrosomas grow ---- they start to convert some of the ammonia to nitrite ---- Nitrobacter grow ---- they start to convert the nitrite into nitrate. Eventually, there is enough bacteria to convert all the ammonia and nitrite in the tank every minute of every day, so that whenever you test for them, the number reads as zero. If you test the water every day during this process, you will see the ammonia build and then "spike" (reach a high point) and then start to go down, and at that time you'll start to see nitrite readings. These will then spike as well, while the ammonia continues to decrease and nitrate will start to register on your test kit. Eventually, the ammonia test readings will hit zero as the bacterial colony has grown to the point where it is consuming all the ammonia produced in the tank almost immediately. The same will happen with the nitrite readings, and as they decrease to zero the nitrate readings will increase. The tank is considered "cycled" when you get readings of zero for ammonia and nitrite, and some reading of nitrate, and this stays consistent day after day with gradual increases in nitrate over time. The water is now safe for fish to live in and will continue to be so as long as you control the nitrates from reaching high levels, usually by doing regular water changes. The entire cycling process can take a few weeks to a couple months. There are several methods of cycling a new aquarium or pond, described in the following posts.