Hospital tank and quarantine tank for fish: setup and how long

A hospital tank (also called a quarantine tank) is a small, simple, separate aquarium where you keep new fish for 4–6 weeks before they join your main tank, and where you treat sick fish. It is the single best way to stop diseases like ich, velvet and worms from reaching your other fish.

It does not need to be fancy. A bare 40-litre (10-gallon) tank, a heater, and a sponge filter that has been living in your main tank are enough. Set it up before you need it, so it is ready on the day you bring fish home or spot a sick one.

What you need

A tank of about 40 litres (10 gallons). Big enough for a few small fish. Use a larger tank for big fish such as adult plecos or cichlids.
A lid. Stressed fish jump. See jumping.
A sponge filter and air pump. Sponge filters are gentle, cheap and easy to clean. The key trick: keep the sponge running in your main tank's filter or sump all the time, so it is full of helpful bacteria and ready to go.
A heater with a guard, and a thermometer.
Hiding places that are easy to clean: a clay pot, PVC pipe sections, or plastic plants. Plecos and catfish need a cave.
No substrate. A bare bottom lets you see poop, uneaten food and dead parasites, and makes cleaning quick. Medicines are not soaked up by gravel.
Its own net, bucket, siphon and test kit, kept apart from your main tank gear.
Dimmer lighting or none. Room light is often enough and calmer for nervous fish.

Step by step

Setting it up

Seed the filter weeks ahead. Put the sponge filter in your main tank (or squeeze it into your main filter) so it grows beneficial bacteria. See cycling.
Fill the hospital tank with dechlorinated tap water, or part water from your main tank. Bring it to the right temperature.
Move the seeded sponge filter in and start the air pump.
Add hiding places and the lid.
Test the water after a day: ammonia and nitrite should be zero.
Acclimate the new fish gently and release them into the hospital tank. See acclimation. Don't pour shop water into either tank.

How long to quarantine

Aim for 4–6 weeks after the last fish was added. Advice varies: university fish-health guides recommend at least 30 days, and the Merck Veterinary Manual suggests 30–60 days for valuable fish. Shorter periods can miss slow problems such as worms and some bacterial diseases.

If you add another fish to the hospital tank partway through, the clock restarts for everyone.

Daily care during quarantine

Look at every fish at least once a day, ideally at feeding time.
Feed lightly and remove uneaten food after a few minutes.
Change 20–30% of the water two or three times a week, or more if ammonia appears. A small tank fouls quickly.
Test ammonia and nitrite every few days.
Wash your hands, or use separate gloves, between the hospital tank and your main tank. Work on the main tank first and the hospital tank last.

Preventive treatment: the debate

Experienced keepers disagree about whether to treat healthy-looking new fish "just in case" (called prophylactic treatment).

For: many fish carry parasites such as flukes and worms without showing signs. Wild-caught fish, including many plecos, very often arrive with internal worms. A standard dewormer course during quarantine catches them early. See internal worms and flukes.
Against: every medicine is a stress, some harm sensitive species, and overuse of antibiotics breeds resistance. Many vets advise watching first and treating only what you find.

A common middle path is: let fish settle and eat for about a week, then treat for parasites with gentle products such as praziquantel, and never use antibiotics without a clear reason. Antibiotics need a vet prescription in many countries anyway.

Using it as a hospital tank

When a fish in your main tank gets sick, move it to the hospital tank to treat it on its own. This has big advantages:

You can dose medicine without harming shrimp, snails, plants or sensitive fish in your main tank.
You use far less medicine.
The sick fish is safe from bullies and easy to watch.

Some diseases, such as ich and velvet, spread through the whole tank. For those, treat the main tank as well, or every fish, because the parasites are in the water too.

Dosing in a bare tank

Measure the real water volume. A "10-gallon" tank usually holds less once it is not filled to the brim. Use a jug or bucket to measure, and dose for the actual water.
No carbon. Sponge filters have none, which is ideal. If you use another filter, remove the carbon, because it pulls medicine out.
Extra air. Many medicines lower oxygen. Keep the air pump running.
Watch the filter bacteria. Some medicines harm them, so test ammonia daily during treatment.
Re-dose after water changes only as the label says.
Sensitive fish: scaleless fish, plecos and other catfish, and loaches are sensitive to copper, formalin, malachite green and some dyes. Use half-strength or gentler choices, and watch closely.

Cleaning it afterwards

Remove all fish.
Empty the tank and scrub it, the heater and all decor with hot water.
Disinfect: soak with a dilute household bleach solution for at least 20 minutes, or use a suitable aquarium disinfectant, following the label.
Rinse very thoroughly, then treat with a strong dose of dechlorinator and let everything air-dry. Dry tanks kill many parasites.
Throw away the sponge after a serious disease such as fish TB. After milder problems, you can disinfect it, but you will need to reseed it in the main tank before next time.
Keep the hospital tank's net and siphon with the hospital tank.

Common mistakes

Setting it up on the day. An unseeded filter means ammonia spikes and sick fish get sicker.
Quarantining for one week. Too short to catch many problems.
Sharing nets and buckets between tanks, which carries disease straight across.
Adding gravel and lots of decor, which hides problems and soaks up medicine.
Treating with several medicines at once. This stresses fish and can be toxic.
Not quarantining plants and snails. They can carry ich, snails and hitchhikers. Keep new plants in a fish-free container for a couple of weeks.

Products that help

Small glass tank with a lid: about 40 litres (10 gallons) for small fish.
Sponge filter and air pump: gentle filtration you can keep seeded in the main tank.
Adjustable heater with guard, and a thermometer: keeps the temperature steady.
Liquid water test kit: small tanks swing quickly, so test often.
Dechlorinator: for frequent water changes.
Dedicated net, bucket and siphon: stop cross-contamination.
Clay pots or PVC pipe: easy-to-clean hiding places.
Praziquantel and a roundworm dewormer: common quarantine treatments for new fish.

Why it matters

Most disease outbreaks in home aquariums start with a new fish, plant or snail. A fish can look healthy in the shop and carry ich, flukes or worms that only show up days or weeks later. Quarantine gives those problems time to appear somewhere they can't reach the rest of your fish.

When to get help

Ask an aquatic vet or experienced fish shop if fish in quarantine keep dying, if you cannot tell what is wrong, or if you are quarantining valuable or wild-caught fish and want a treatment plan.

In the game

Multiple Tank Syndrome has a "Quarantine tank" card, and this guide is the real-life version.