How to Set Up a Simple Quarantine Tank (And Why You Must)

Here's the uncomfortable math of skipping quarantine: marine ich and velvet are common in the supply chain, a single infected fish can seed your display with parasites, and once a parasite is in the display, the only cure is removing every fish to a hospital tank while the display sits fishless for six weeks or more. One $20 fish, added directly, can cost you your entire livestock collection and two months of the hobby.

Against that, a quarantine tank costs about $80 and takes an afternoon to set up. It is the best-value insurance in reefkeeping, and — this is the part nobody tells beginners — it's supposed to be ugly. No sand, no rock, no light schedule to speak of. The bar is low. Let's clear it.

What quarantine actually accomplishes

A quarantine (QT) tank does three jobs:

1. Observation. Most diseases show themselves within 2–4 weeks. In a bare tank you can actually see the fish daily — spots, breathing rate, feeding response — instead of losing it into the rockwork.
2. Isolation. Whatever the new fish carries, it can't reach your display, your existing fish, or your inverts.
3. Treatment without collateral. Copper — the reliable treatment for ich and velvet — is lethal to corals, inverts, and the beneficial life in your rock and sand. You can never treat a reef display. A bare QT is the only place medication is even possible.

There's a quieter fourth benefit: a QT gives shy, shipped-stressed fish a few calm weeks to eat aggressively and recover condition before facing tankmates. Fish routinely enter the display stronger than they left the store.

The $80 build (shopping list)

• A 10-gallon glass tank (~$20, or free from any secondhand marketplace). Twenty gallons if you plan to QT larger or multiple fish.
• A sponge filter + cheap air pump (~$15) — or a small hang-on-back filter.
• A heater (~$20). QT is exactly where budget heaters go — but if this tank will double as your emergency/hospital tank for years, a heater with a controller is the same cheap insurance it is on the display.
• A lid (~$10). Stressed new fish are jumpers. Non-negotiable.
• PVC elbows or small sections of pipe (~$5). These are the "rockwork" — hiding places with no biology and no medication absorption.
• A thermometer (~$5).
• No sand. No rock. No light beyond the room's. Bare glass bottom — it lets you see waste, uneaten food, and (if treating) keeps medication levels stable. Substrate and rock absorb copper and make dosing a guessing game.

Total: roughly $75–100. It stores in a closet between uses.

The biofilter problem (and the honest solutions)

A QT is a real tank — ammonia is still the enemy, and a bare 10-gallon has no established biofilter. Three workable approaches, best first:

1. Keep a sponge filter running in your display's sump or back chamber permanently. When a new fish comes home, move the sponge to the QT — instant cycled tank. This is the pro move and costs nothing.
2. Cycle the QT with bottled bacteria a couple of weeks before the fish arrives, same as cycling any tank.
3. Run it as a semi-sterile tank with water changes — large (25–50%) changes every day or two, using display water or fresh saltwater, with an ammonia badge stuck to the glass and a test kit backing it up. Labor-intensive but effective for a single small fish.

Whichever route, test ammonia frequently — a reef test kit with a liquid ammonia test is part of the QT kit, and an adhesive ammonia alert badge ($8) is a worthy supplement, since it watches the tank between tests.

The protocol: 30 days, watch and feed

The simplest defensible protocol for beginners is observation quarantine:

1. Day 0: Acclimate the new fish to the QT — drip acclimation, covered step-by-step in our drip acclimation guide. Keep lights off and the room calm for 24 hours.
2. Days 1–30: Feed daily, observe daily. You're watching for: white spots (ich), fine gold dust and rapid breathing (velvet), sloughing slime (brooklynella), pinched belly, refusal to eat past day 3, cloudy eyes, frayed fins. Keep water pristine per your biofilter plan.
3. Any symptoms: identify before treating — velvet and ich both respond to copper at therapeutic levels held for 14+ days; brooklynella wants formalin. Diagnosis matters. Photograph the fish and consult a disease forum or your LFS before dosing anything.
4. Day 30, healthy fish: match the QT's salinity to the display over a few days if they've drifted apart, then move the fish. Done.

Many experienced keepers run prophylactic copper on every fish regardless of symptoms — a reasonable graduate-level protocol, but copper dosing has a learning curve (stable levels, testing, duration) and observation-QT catches the large majority of problems with zero medication risk. Start with observation; adopt prophylaxis when you're comfortable.

One scope note: inverts and corals don't share fish diseases — snails and shrimp don't need fish QT (though corals deserve their own dip-and-inspect protocol at the door). And everything wet from another tank can, in principle, carry parasite cysts in residual water; keep QT nets, buckets, and hands separate from display gear, or rinse in freshwater between tanks.

The habit that makes it stick

The reason QT protocols fail isn't complexity — it's impulse purchases. The fish store doesn't sell patience, and a gorgeous fish with no QT waiting at home becomes a direct-to-display gamble. So flip the sequence: the QT gets set up before the shopping trip, every time. Fish shopping is something you do with a running quarantine tank, the way you buy groceries with a working refrigerator.

Stock slowly, quarantine everything, and your display can genuinely go years without a disease event — the fish you skip and the shortcuts you refuse matter more than any medication. That philosophy, plus the full first-fish sequence, is covered in our beginner's guide, The First Tank.

FAQ

Do I really need a quarantine tank for my very first fish?

The first fish is the one genuine exception — it enters an empty display, so there's nothing to infect and the display is the observation tank. Enjoy the freebie: every fish after that one goes through QT, because now there's something to lose.

How long should quarantine last?

Thirty days symptom-free is the standard observation window — long enough for ich and velvet to reveal themselves at normal temperatures. If anything appears and you treat, the clock restarts after treatment ends. Two weeks is better than nothing; thirty days is the number that catches slow-developing infections.

Can I quarantine two fish at once?

Yes, if they're compatible and the tank has room — in fact QTing tankmates together (a pair of clownfish, say) reduces stress and saves a cycle. Don't stack sequential purchases though: adding a new fish to an occupied QT restarts the 30-day clock for everyone, since the newcomer could infect the resident.