What a fish-in cycle asks of you
Cycling is the process of growing the bacteria colonies that convert toxic ammonia from fish waste into nitrite, and then nitrite into far less harmful nitrate. Doing it with fish already in the tank means those pioneers live in the very water you are trying to detoxify, so the safety margin comes from you, not from the bacteria. Your job for the whole cycle is dilution duty: test daily, and whenever ammonia or nitrite starts to climb, change water to bring it back down. Done faithfully, the colonies still establish on the filter media and surfaces; they just do it while you hold the water at levels the fish can tolerate.
Set the tank up to give the fish a margin
Start with far fewer fish than the tank will eventually hold, and choose genuinely hardy species that forgive beginner water: zebra danios and platies have long been the traditional pioneers for good reason. If you know anyone with an established tank, a squeeze of their filter media or a handful of their gravel carries mature bacteria and can shorten the risky weeks considerably; bottled bacteria starters aim at the same shortcut. Run the filter from day one and never rinse its media under the tap, because chlorinated water kills the colonies you are growing. Keep the temperature steady in the normal range for the species, since bacteria multiply faster warm than cold.
The daily rhythm while bacteria establish
Test ammonia and nitrite every day with a liquid kit and write the numbers down; test strips are too vague for the small rises you are trying to catch. The pattern you are watching for is ammonia rising first, then falling as nitrite rises, then nitrite falling in its turn while nitrate appears. Feed once a day, lightly, and skip a day without guilt if a reading spikes: hungry fish produce less waste, and uneaten food rotting in the gravel is ammonia you added yourself. Change a portion of the water whenever either toxin registers clearly, using dechlorinated water at the tank's temperature, and retest a few hours later to confirm the level dropped. An ammonia-binding water conditioner can buy extra safety between changes during a stubborn spike.
Resist the two classic mistakes
The first is adding more fish because the pioneers look fine. They look fine because the load is small; each new arrival raises waste faster than the young colony can grow, and the tank tips back into danger. The second is deep-cleaning: scrubbing algae, replacing filter media and vacuuming every corner at once removes bacteria colonies wholesale. During a cycle, clean only what blocks flow, leave the media alone, and accept a tank that looks a little untidy. Resist also the urge to chase a single odd reading with panic measures; retest first, change water second, and let the trend across several days guide you.
Knowing when the cycle has finished
The cycle is done when ammonia and nitrite both read zero day after day while nitrate slowly accumulates, with the fish eating eagerly and behaving normally. Only then add fish, a few at a time, with a week or more between groups, and keep testing for a few days after each addition because each group asks the colony to grow again. Keeping the whole cycle's readings together in the tank's KinsLe Pet Passport gives you the proof of when it finished and a baseline of normal numbers to compare against whenever something looks off months later.