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 species convert ammonia (NH₃) to nitrite (N02-), while others convert nitrite to nitrate (NO3-). Thus, cycling the tank 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 tank, 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 tanks), zebra danios and barbs for warmer tanks, and damselfishes in marine systems. Note: Do not use "toughies'' or other feeder fishes. Although cheap, they are extremely unhealthy and using them may introduce unwanted diseases to your tank.