A plain tank with everything it needs
Quarantine works because it is boring: a small tank, somewhere between a large bucket's worth and a modest desktop aquarium, with no gravel, no other fish and nothing shared with the main display. It still needs the essentials - a cycled sponge filter, a lid because white clouds jump, a simple hide so the fish can settle, and water matched to what it came from. The filter is the part people skip and regret: an uncycled quarantine tank spikes ammonia within days and makes a healthy fish ill. Run the sponge in your main tank for a few weeks beforehand, or seed it with media from the established filter, so the quarantine tank is biologically ready before the minnow arrives.
Cool water, gentle filtration
White cloud mountain minnows are cool-water fish from mountain streams, and quarantine should respect that rather than defaulting to tropical settings. Room-temperature water in the mid-sixties to low seventies Fahrenheit suits them, so in most homes the quarantine tank needs no heater at all - just a thermometer to confirm it is not sitting somewhere cold or sun-baked. The sponge filter's soft flow suits a small fish finding its feet, and gentle aeration keeps oxygen comfortable. Feed lightly once a day; a new fish in a bare tank fouls the water quickly if you feed generously out of kindness.
The daily watch list
The point of the stay is observation, so look properly once a day with the tank light on. A healthy white cloud feeds eagerly, holds its fins open, and shows clean edges to the red-and-white finnage. What you are watching for is change: pinhead white spots scattered over body and fins, a clamped tail, flicking or rubbing against the hide, cottony patches, or a fish that hangs at the surface or ignores food. Do not treat 'just in case' if none of these appear - unnecessary treatments stress a healthy fish and can mask the very signs you are waiting to read. If signs do appear, that is the moment to seek advice from a vet who treats fish or an experienced aquatic health professional, with your observations ready.
When quarantine extends, and when it ends
Two to four weeks of clean health is the standard stay; if anything on the watch list shows up, the clock restarts from when the fish is clear again, because a quarantine that ends on a schedule rather than on evidence is no quarantine at all. Keep nets and siphons for this tank separate, and always work on the main tank first and the quarantine tank last, so your own hands do not carry anything between them. When the stay ends well, match temperatures between the tanks, move the fish across, and give it a quiet first day in the main tank's planting before judging how it has settled.
A health history that travels with the fish
A quarantined fish arrives in your main tank with something valuable: a known recent past. The arrival date, the shop it came from, what you observed each week and how it fed are exactly the details that matter if it ever looks unwell later - and exactly the details that evaporate from memory within a month. Jotting the quarantine record into the minnow's Pet Passport on KinsLe keeps that history attached to the fish for good, so its clean bill of health is documented rather than remembered, and any future question about it starts from facts.