Like all living creatures, fish give off waste products (pee and poo). These nitrogenous waste products break down into ammonia (NH3), which is highly toxic to most fishes. In nature, the volume of water per fish is extremely high, and waste products become diluted to low concentrations. In aquariums, however, it can take as little as a few hours for ammonia concentrations to reach toxic levels
In aquaria-speak, the “nitrogen cycle” (more precisely, the nitrification cycle) is the biological process that converts ammonia into other, relatively harmless nitrogen compounds. Fortunately, several species of bacteria do this conversion for us. Some tropical fish species convert ammonia (NH3) to nitrite (N02-), while others convert nitrite to nitrate (NO3-).
Thus, cycling the aquarium refers to the process of establishing bacterial colonies in the filter bed that convert ammonia -> nitrite -> nitrate. The desired species of nitrifying bacteria are present everywhere (e.g., in the air). Therefore, once you have an ammonia source in your aquarium, it’s only a matter of time before the desired bacteria establish a colony in your filter bed.
The most common way to do this is to place one or two (emphasis on one or two) hardy and inexpensive fish in your aquarium. The fish waste contains the ammonia on which the bacteria live. Don’t overfeed them! More food means more ammonia! Some suggested species include: common Goldfish (for cold water aquariums), zebra danios and barbs for warmer aquariums, and damselfishes in marine systems.
Because nitrate-forming bacteria don’t even begin to appear until nitrite is present in significant quantities, nitrite levels skyrocket (as the built-up ammonia is converted), continuing to rise as the continually-produced ammonia is converted to nitrite. Once the nitrate-forming bacteria take hold, nitrite levels fall, nitrate levels rise, and the aquarium is fully cycled.
Your tank is fully cycled once nitrates are being produced (and ammonia and nitrite levels are zero). To determine when the cycle has completed, buy appropriate test kits.
The cycling process normally takes anywhere from 2-6 weeks. At temperatures below 70F, it takes even longer to cycle an aquarium. In comparison to other types of bacteria, nitrifying bacteria grow slowly. Under optimal conditions, it takes fully 15 hours for a colony to double in size!