You bring the tank home, fill it, switch on the filter, and the water looks crystal clear. It feels ready. It isn't. The water is clear, but it has no helper bacteria yet, and without them the first waste your fish make slowly poisons the tank. Getting those bacteria to move in is called cycling, and it is the single most useful thing a new fishkeeper can learn.
Fish breathe out and pass ammonia, and leftover food and droppings rot into more of it. Ammonia is toxic: it burns gills and makes breathing hard. In the wild it is swept away by the river. In a glass box it just builds up.
Luckily, two groups of bacteria quietly settle on every surface in the tank, mostly inside the filter, and clean up in two steps:
A new tank starts with almost none of these bacteria. They only grow once there is ammonia to eat, which is why the process takes weeks and cannot be rushed by waiting with an empty tank.
Drag the slider to move through the weeks. The curves are a simplified picture of a typical fishless cycle, not a forecast for your tank: real tanks differ in timing and height, so always use a test kit.
Sources give a range, and it depends on temperature, the water and how the tank is set up. The RSPCA in the UK says there is no exact rule but at least two weeks; Practical Fishkeeping says usually three or four weeks, sometimes more; INJAF and the RSPCA in Australia both say four to six weeks. Plan for about a month and let the test kit, not the calendar, tell you when you're done.
Warmth helps, because the bacteria grow faster in warmer water, and so does a bit of used filter sponge or media from a healthy, disease-free established tank (ask a friendly fishkeeper or a good local shop). Bottled bacteria products may speed things up, but you still need to test.
There are two ways to run a cycle. In a fishless cycle you add a source of ammonia (for example pure ammonia without perfumes or additives, or a pinch of fish food) and let the bacteria grow with no animals in the tank. The RSPCA in the UK strongly advises this, and so do most fishkeeping groups. In a fish-in cycle a few hardy fish make the ammonia, which means they live through the spikes. Some vets describe managing this with small stocking and frequent water changes, but it is stressful for the fish, so fishless is the kinder route.
The tank is ready when ammonia and nitrite both read zero and nitrate is showing up.
If ammonia or nitrite reads above zero with fish in the tank, a partial water change with conditioned water is the usual first step (the RSPCA suggests changing around half the water daily until it improves), then keep testing. Fish gasping at the surface, clamped fins or sudden listlessness are reasons to call an aquatic vet or a knowledgeable fish shop. This guide can't diagnose a sick fish.