How to quarantine new and sick fish (the QT tank)

The whole point of quarantine is isolation, and the reason a hospital tank works is that you can medicate it without consequences. A 10 to 20 gallon tank does both. Ten gallons is plenty for most single fish or a small group of the same species; step up to 20 if you're holding a tang, a larger cichlid, or several fish at once.

Keep it bare-bottom on purpose. No gravel, no sand, no live rock, no plants. Substrate and rock trap medication, soak up copper, and give parasites places to complete their life cycle out of reach. A bare glass bottom is easy to see waste on, easy to siphon, and easy to disinfect when you're done. The only furniture is a length of PVC pipe or a couple of plastic fittings so the fish has somewhere to feel hidden. Less stress means a fish that keeps eating, and a fish that keeps eating is a fish that recovers.

Think of it as disposable infrastructure. When treatment's over, you drain it, bleach it, rinse it well, dry it, and store it flat until the next time.

A QT still needs to be cycled, or the ammonia from a stressed, possibly-sick fish will finish what the disease started. The easy way is a sponge filter that's been sitting in your display or sump for a few weeks, quietly growing bacteria. When you need the QT, you pull that seasoned sponge, drop it in the quarantine tank, and you've got an instant biological filter.

The habit to build: always keep a spare sponge filter running in your main system. It costs you nothing once it's in there, and it means you're never scrambling to cycle a tank the day you bring a fish home. If you didn't plan ahead, you can still run the QT and lean on water changes plus a bottled bacteria starter, but you'll be testing ammonia daily and changing water often. See our guide on cycling a new aquarium if the nitrogen cycle is new to you.

One catch worth knowing: copper and many medications will kill the bacteria in that sponge. If you medicate, expect to manage ammonia by hand for the duration and re-seed afterward. That's normal. It isn't a sign you did anything wrong.

Fill the QT with water that matches the tank the fish is going into, or matches our shop water if it's a new arrival from us. Same temperature, and on saltwater the same salinity. A fish that's already stressed from shipping or illness does not need a salinity or temperature swing on top of it.

Get the heater in and stable before the fish goes in, and keep a thermometer on the glass so you're reading the actual water, not the heater's dial. For most tropical fish you're looking at roughly 76 to 80°F; we can confirm the right range for your specific species. Size the heater with a little headroom, because treating freshwater ich means raising the temperature to around 86°F, and an undersized heater can't get a 20-gallon tank there. Our guide on sizing your filter, heater and lighting covers the wattage math; for a QT that may need to hit treatment temps, plan on roughly 5 watts per gallon.

Lighting is optional and honestly best kept dim. A QT isn't for display, and lower light keeps a nervous fish calmer. Skip the fancy fixture; a low ambient room light is plenty.

If the fish looks healthy, you're not treating anything yet. You're observing. Feed normally and watch for trouble: scratching or flashing against objects, white spots like grains of salt, clamped fins, rapid breathing, fuzzy patches, frayed fins, or a fish that won't eat. Many diseases that hitchhike in on a new fish only show themselves once the fish settles and the stress of shipping wears off, which is exactly why you wait.

Two to four weeks is the working window for most freshwater fish. On the saltwater side, give it longer. Marine ich (Cryptocaryon) can sit latent for weeks, so a lot of experienced reef keepers run new fish through a minimum of four to six weeks before they trust them near a display. Pulling a marine fish early is how an outbreak ends up in the reef you spent a year building.

Keep up small, frequent water changes during this period, since a bare tank with no substrate has limited buffer. Test ammonia and nitrite the way you would on any uncycled or lightly-cycled tank, and keep them at zero. If the fish sails through the whole window clean, eating, and acting normal, it's earned its spot. Acclimate it into the display the same careful way you would any new fish. If something shows up, you're already exactly where you want to be to deal with it.

Marine ich (Cryptocaryon irritans) is the classic reason reef-keepers swear by quarantine. It shows as fine white spots and flashing, and it spreads fast in a closed system. There are two well-established approaches, and both happen in the QT, never the display.

The first is copper at therapeutic levels, held steady for the full course while you test copper concentration daily with a dedicated test kit. Copper has a narrow window: too little doesn't work, too much poisons the fish, so measuring is not optional. The exact target level and duration depend on the product, so follow the label, and stop in if you want to walk through it before you start. The hard rule with copper: never expose invertebrates, corals, snails, crabs, or live rock to it. It kills them and lingers. Copper belongs only in a bare QT.

The second approach is the tank-transfer method, which uses no medication at all. You move the fish between two clean, fully-separate setups on a fixed schedule that breaks the parasite's life cycle before it can reinfect. It's labor-intensive but invert-safe and very effective when done precisely. Other saltwater issues like marine velvet, brooklynella, or bacterial infections each call for a different treatment, so the first move is always to identify what you're actually looking at. Getting the diagnosis right is most of the battle, and we're glad to help you ID it from a clear photo or a description of the behavior.

Freshwater ich (Ich, Ichthyophthirius multifiliis) looks similar, small white spots and flashing, but it's a different parasite and the treatment is different. The classic, well-proven approach is heat: raise the temperature gradually to around 86°F (30°C) and hold it there for about ten days, which speeds the parasite through its life cycle to a stage you can clear. Heat alone at that temperature, sustained, is enough to clear ich in many setups without any medication at all. You can add an ich medication alongside the heat for a stubborn case or a heavily-infected tank, but it isn't automatically required, so don't reach for a bottle by reflex.

Watch the fish's tolerance for the heat the whole way up, and add extra aeration, because warm water holds less oxygen. Not every fish tolerates 86°F or every medication. Some scaleless fish and certain species are sensitive to common ingredients like malachite green, so read the label for species cautions and dose to the actual tank volume rather than a guess.

Other freshwater problems, fin rot, fungus, columnaris, internal parasites, each have their own medication, and a broad-spectrum 'cure-all' often isn't the right tool. Match the med to the disease. Send us a clear photo or describe the behavior and we'll point you at the right treatment rather than have you throw three medications at the water and hope.

The most common quarantine mistake we see isn't skipping QT, it's reaching for every bottle on the shelf at once. Medications interact, some cancel each other out, some compound toxicity, and a tank full of mixed meds is harder on the fish than the disease was. Diagnose first, treat one thing deliberately, finish the full course even if symptoms fade early.

Dosing is always to the actual water volume of the QT, and a bare 10 or 20 gallon tank holds less than its rated size once you account for displacement and a partial fill. Underdosing breeds resistant parasites; overdosing poisons the patient. Follow the product label for the dose, the duration, and any species warnings. We're not going to print a dosing chart here because it varies by product and by what you're treating, and a wrong number is worse than no number.

If you can't confidently name what you're treating, stop and ask. Bring a photo, your water numbers, or a sample in and we'll test it for you. We'd rather spend ten minutes getting the diagnosis right than sell you the wrong cure.

After a fish clears quarantine or finishes treatment and goes into the display, fully reset the QT before the next use. Drain it completely, scrub it with a dilute bleach solution, rinse thoroughly, and let everything dry fully. The PVC, the net, the heater, the hardware: anything that touched a sick fish gets disinfected so you're not carrying a problem forward to the next arrival.

If you ran copper, the sponge filter media is spent for biological purposes and copper can cling to it, so plan to re-seed a fresh sponge from your main system rather than reuse a copper-soaked one. Nets and siphons used in the QT should never go back into your display without disinfecting and drying first, since cross-contamination on shared equipment quietly undoes the whole point of keeping a quarantine tank in the first place.

Store the empty tank, heater, and hardware together so it's a fifteen-minute setup next time. The shops and hobbyists who never seem to have disease outbreaks aren't lucky. They just keep a QT ready to go and have the discipline to use it.