New fish — even healthy-looking ones from a reputable source — should spend time in a separate quarantine setup before joining your main tank. This isn’t about distrust of the seller, it’s about not gambling your whole tank on one unknown.
A bare 10-20 gallon tank with a sponge filter, heater, and a hide is enough. Bare-bottom makes it easy to spot parasites or unusual waste, and a sponge filter avoids introducing untested substrate to your main tank later.
Quarantine for two to three weeks minimum, watching for clamped fins, unusual spots, or refusal to eat past the first couple of stress-adjustment days. Have a broad-spectrum medication on hand before you need it — treating early is far easier than treating an outbreak that’s already spread.
Only move fish to the main tank once they’ve eaten normally and looked clean for at least a week straight, not just a day or two of improvement.