Cycling a new tank
A new tank has water and no bacteria. Fish make ammonia, and ammonia is poison. Cycling is the weeks it takes for the right bacteria to settle and clean it up.
The cycle in three steps
Fish waste, uneaten food and rotting plant matter release ammonia. One group of bacteria turns ammonia into nitrite, which is also toxic. A second group turns nitrite into nitrate, which is far less harmful and is removed by water changes and by plants.
The bacteria live on surfaces, mostly inside the filter media. They do not float in the water, which is why a rinsed filter or a brand-new one starts the whole process again.
Fishless cycling
Add a source of ammonia without any fish: pure ammonia from a hardware shop with no additives, or a pinch of fish food left to rot. Test the water every few days. Ammonia rises, then falls as nitrite appears, then nitrite falls as nitrate rises.
The tank is cycled when it can clear a dose of ammonia to zero, and the nitrite with it, inside about a day. That usually takes four to eight weeks, longer in a cool room.
Cycling with fish
It can be done gently with a few hardy fish, small daily feeds and frequent water changes, but the fish carry the risk. If you go this way, start small and test often. Never fill the tank on day one: the planner's Load verdict assumes a mature tank, not a new one.
When it is safe to add fish
Zero ammonia, zero nitrite and some nitrate on a test kit. Then add fish a few at a time, with a week or two between additions, so the bacteria can grow to match the new load.
Check the full list in the planner before buying, and the water and group needs on each fish page.