What does it mean to cycle an aquarium?

Cycling an aquarium is about growing helpful bacteria that process fish waste. Fish poop and leftover food break down into ammonia, which is toxic even at low levels. During a cycle, bacteria build up that convert ammonia into nitrite (also toxic), and then into nitrate, which is much safer and can be controlled with water changes and live plants. A tank is considered "cycled" when it can handle a full day’s waste without harmful spikes. Using a liquid test kit, you can observe ammonia and nitrite rise before both drop to 0 ppm, with nitrate appearing (typically between 5-40 ppm). Most new setups usually take about 3-6 weeks to cycle. It can be quicker if you use established filter media, but it might take longer in cold tanks or if the filter has limited surface area. Most people cycle their tanks fishless (by adding an ammonia source and testing every few days for safety) or with a few fish (by lightly feeding and doing frequent water changes to keep ammonia and nitrite around 0.25 ppm). Once you're cycled, try not to replace all your filter media at once or deep-clean everything the same day, because that's where a lot of your bacteria live.