Also referred to as the biological cycle or startup cycle, the “cycling” is the colloquial term for a natural process that occurs in every new aquarium. Lasting for approximately 1-2 months, the cycle begins as soon as fish (or any other source of ammonia) is introduced to the tank.
After the cycle has finished, an aquarium is refered to as “cycled,” meaning that it now has large enough quantities of beneficial bacteria to keep it stable.
A closely related term, the nitrogen cycle is a never-ending cycle in which nitrogen and its compounds are passed through the environment and living organisms. In an aquarium context, the “cycling” of an aquarium almost always refers to the establishing of beneficial bacteria that help convert ammonia into nitrates.
All organic material in an aquarium eventually decays and produces ammonia or ammonium (depending on your water's pH), both of which are toxic to all fish. If the pH of your aquarium water is less than 7, organic material will convert into the much less toxic ammonium (NH4+). If your water's pH is above 7, it will become ammonia (NH3), the more toxic form.
Once ammonia has built up, beneficial bacteria that remove ammonia/ammonium will begin to colonize your tank. These bacteria release nitrite, another chemical compound which is also poisonous to fish. After a few more weeks, populations of nitrite-eating bacteria will begin to establish themselves, producing nitrate (with an a) as their waste product. Nitrate isn't nearly as harmful to fish as its cousins, ammonia and nitrite, but is still harmful to fish at higher levels.
Nitrate can be removed from the aquarium by water changes and through plants. Even though plants can remove nitrates in addition to small amounts of ammonia, water changes are still a necessity. When aquarium water is not replaced, certain essential trace minerals will be depleted, and other types of organic wastes will build up as well. Having plants, better filters, and a smaller bio-load can reduce the time between water changes, but they [sadly] do not eliminate the need to perform them altogether.
The method of cycling an aquarium can be subdivided into two categories: fish-in and fishless. As their names imply, fish-in cycling occurs while fish are in the tank and fishless cycling happens without the fish.
Personally, I find the fishless cycle to be more humane as fish will inevitably have toxins in their water with a fish-in cycle. If you are a beginner and you don't already have your fish, please consider a fishless cycle, as keeping the amount of toxins at a minimum while still cycling the aquarium can be a challenge. If you already have your fish, make sure you test your water parameters (ammonia, nitrites, nitrates) with a test kit daily while cycling.
Possibly even more controversial than the fish-in v.s. fishless cycle is skipping the cycle altogether. This is usually done through bacteria in a bottle, freshwater live sand, or moving used filter media. These methods will not “skip” the cycle per say, but they can definitely speed up the process.
Thanks for reading this short exploration into the nitrogen cycle (last modified: 10/15/16)! For more information, please browse around AquariumKids.com! Feel free to contact me at evanb [at] aquariumkids [dot] com with any questions.
Special thanks to Allison Mou for helping proofread/edit this page!