Cycling your tank (the nitrogen cycle) without myths

Fish excrete ammonia, which is toxic. Beneficial bacteria convert it to nitrite (also toxic), and other bacteria convert nitrite to nitrate, which is much less harmful and is removed by water changes and plants.

These bacteria live mostly on surfaces, especially in the filter, and a new tank needs time to grow them. That is “the cycle”.

Fishless: you add an ammonia source to the water and use tests to see when ammonia and nitrite fall to zero with nitrate present. It is considered kinder to fish.

Fish-in: you start with very few hardy fish and change water very often. It works, but is more stressful for the fish and needs careful monitoring. Aquarists disagree about which is better.

Typically a few weeks, often 4–8, but it can be quicker or slower. So go by tests, not the calendar: it is ready when ammonia and nitrite stay at zero and nitrate appears.

“The water is clear, so it is ready.” No: clear water can be full of ammonia.

“Bottled bacteria solve everything.” They may shorten the process but do not replace testing and adding fish cautiously.

“The filter should be cleaned spotless.” Rinsing under the tap kills the bacteria. Rinse it in water taken from the tank.