Why do I need a quarantine tank, and how do I set one up?

Short answer: A quarantine tank (QT) is a separate, bare-bones tank where every new fish lives alone for 3–4 weeks before it ever touches your display — long enough to see and treat diseases like ich or velvet before they reach a tank you can't safely medicate. Skipping QT is the single most common way a new reefer loses an entire tank to disease.

The details

Why it matters. Display reef tanks hold corals and invertebrates that most effective ich and velvet medications (copper, some dewormers) will kill. Once ich (Cryptocaryon irritans) or velvet (Amyloodinium) reaches the display, you usually can't treat it there without also wiping out everything else. Quarantine is what lets you treat a fish aggressively, if needed, without risking the rest of the tank.

What you need. A separate small tank, a heater, a simple filter (hang-on-back or sponge), PVC pipe fittings for hiding spots (a bare tank stresses a new fish), and a screen lid — stressed new fish jump.

How long. 21–28 days (3–4 weeks) of observation is the standard duration in our sources — long enough for ich and velvet's life cycle to show itself if it's present.

What happens in QT. Watch daily for the disease signs from the previous card (white spots, labored breathing, refusal to eat). Some reefers also run a preventive treatment during this window — our sources mention PraziPro plus Copper Power as one commonly used combination. Always follow the product's own label instructions, and if you're dosing copper, use a copper test kit/checker rather than dosing by eye — too little under-treats, too much is lethal to fish.

Corals need a version of this too. When you eventually add corals (a later stage), the same logic applies — dip and quarantine before anything touches the display. That's a separate topic for when you get there.

Clean-up crew is different. Snails and hermits generally aren't run through the same fish-disease quarantine process in our sources — careful drip acclimation (previous card) is what matters most for them.

The numbers

Common mistakes

• No QT at all — "it looked healthy at the store" is not quarantine.
• Cutting QT short because the fish "looks fine" after a week — some diseases take longer to show.
• Medicating the display tank instead of QT.
• Running an uncycled QT without watching ammonia/nitrite there too — a QT tank can mini-cycle just like the display did.

When to worry

• Normal: a new fish hiding and not eating for the first day or two in QT.
• Worry: white spots appearing, labored breathing, or no eating by day 3–4 — that's exactly what QT is for; treat there, not in the display.

What's next

With disease risk handled, it's time to pick which species actually make good first additions: best-first-fish-for-a-reef-tank.