The single most important step before adding fish — explained simply.
Cycling means growing the beneficial bacteria that turn toxic fish waste into something far less harmful. Skip it and ammonia spikes will stress or kill your fish — this is the #1 reason new tanks fail.
Fish produce ammonia (toxic). One group of bacteria converts ammonia to nitrite (still toxic). A second group converts nitrite to nitrate (much safer), which you remove with water changes. A 'cycled' tank has enough of both bacteria to keep ammonia and nitrite at zero.
Add a source of ammonia (bottled ammonia or a pinch of fish food) to an empty, filtered tank and test daily. When ammonia and nitrite both read zero within 24 hours of dosing, and you see nitrate, you're cycled — usually 3–6 weeks. A bottled bacteria starter and a temperature around 80°F speed it up.
Add ammonia to about 2 ppm in the evening. If by the next evening both ammonia and nitrite read 0 ppm, the tank can handle a full fish load. Do a large water change before adding fish to drop the accumulated nitrate.