The nitrogen cycle is the biological process that makes a healthy aquarium possible. When fish produce waste, uneaten food decomposes, and organic material breaks down, ammonia is released into the water — and ammonia is toxic to fish even at very low concentrations.

Beneficial bacteria that colonize your filter media and substrate convert ammonia into nitrite (also toxic), and then a second type of bacteria converts nitrite into relatively harmless nitrate. This entire process — known as 'cycling' — typically takes 4 to 6 weeks in a new aquarium.

There are several ways to cycle a tank. The 'fishless cycle' method involves adding ammonia directly (from a pure ammonia source) to feed the bacteria without risking fish lives. The 'fish-in cycle' uses hardy fish to provide ammonia, but requires very frequent water changes to keep levels safe. The fastest method uses bottled beneficial bacteria products, which can significantly speed up the process.

During cycling, you should test your water daily using a liquid test kit (more accurate than strips). You're looking for: ammonia to rise then drop, nitrite to rise then drop, and nitrate to appear. Once ammonia and nitrite both read 0 ppm and nitrate is detectable, your tank is cycled and safe for fish.