Quarantine is the cheapest insurance in fishkeeping: a plain spare tank where new arrivals live under observation before they can share water with the fish you already have. Shop tanks mix fish from many sources, and stress from capture and transport lowers a new fish's defences just as it arrives with you. The stay is not really about the calendar — it is about giving hidden problems time to show themselves where they can only harm one fish, not a whole community.
Setting up a quarantine tank that works
Keep it simple and stable: a bare-bottomed tank with a cycled sponge filter, a heater matched to the species, a lid, and one or two plain hiding places so the fish can settle without you needing to furnish it like a display. Bare glass makes waste visible and cleaning quick, and a sponge filter carries the bacteria a small tank needs without strong currents. Dim light for the first day or two lowers stress. The quarantine tank's water should match your main tank's temperature and chemistry closely, so that release day, when it comes, is a short move rather than a second shock.
What the weeks are actually for
Watch feeding first: a healthy new fish usually shows interest in food within a day or two, and steady appetite across the stay is one of the strongest good signs. Then look at the body daily in good light — fins held open rather than clamped, skin free of spots, dusting or patches, eyes clear, breathing even rather than rapid. Behaviour matters as much: fish that rub themselves on surfaces, hang at the top gasping, or sit apart and listless are telling you the stay needs to be longer and the cause found. Two weeks is the common minimum because many problems surface within it; four weeks gives slower-developing issues their chance to appear and is the wiser span for wild-caught fish.
What restarts the clock
The quarantine period runs from the last worrying event, not from arrival day. If white spots, fungus-like patches or any other symptom appears in week two, the clock resets to the day the fish is finally clear and feeding normally again. Adding a second new fish to the same quarantine tank in week three does the same thing, because the newcomer may carry something the first fish has not yet met — both fish then need the full observation window from that arrival. This is why a simple written log beats memory: dates blur quickly when a tank looks fine.
Release day, and what the record gives you
When the full period passes with clean observations, match the quarantine water to the main tank gradually if any drift has crept in, then move the fish with a net and a quiet room. Record the arrival date, each day's brief observation and the release date in the fish's Pet Passport on KinsLe, and that page becomes the start of this fish's health history — the reference you will want later if anything in the main tank ever raises a question about when a problem began.