How to set up a quarantine tank, and why every disease guide sends you here

The tank that prevents most of the other guides

Nearly every disease that arrives in an aquarium arrives in a bag. A fish from the shop has spent the week sharing water with hundreds of others from a dozen suppliers, and ich, velvet, flukes, fin rot and internal worms all travel that way, invisibly, on a fish that looks perfectly well for ten days. A quarantine tank is where those ten days happen somewhere other than your main tank.

It does a second job just as well. A sick fish in a community is hard to medicate, because the treatment goes into 200 litres and reaches the plants, the shrimp and the fish that were fine. The same fish in a bare 20-litre hospital tank gets the full dose, a daily water change that takes two minutes, and no tankmates to harass it. Keepers who own one use it far more for this than for new arrivals.

It costs about the price of one lost centrepiece fish, and it can live empty in a cupboard between uses.

A quarantine tank, ready in an evening

• ✓20 to 40 litre tank with a lid
• ✓Sponge filter, kept running in the main tank between uses
• ✓Heater with a thermostat, and a thermometer
• ✓A hiding place: a pipe, a pot or a plastic plant
• ✓Bare bottom, no plants, no decor
• ✓Its own net, bucket and siphon
• ✓Filled from the main tank, the day before

What you need

• A 20 to 40 litre tank, glass or plastic, with a lid. A plastic storage box works. It does not need to be pretty; it needs to hold water and not be jumped out of.
• A sponge filter on an air pump. Gentle, cheap, and the important part: keep the sponge running in your main tank’s filter or sump the rest of the time, so it is already full of bacteria the day you need it. A quarantine tank set up with a fresh sponge is an uncycled tank, and you will be fighting new tank syndrome on top of whatever the fish brought.
• A heater with a thermostat, sized for the tank.
• A thermometer, and a liquid test kit, which you already own.
• A hiding place. A short length of PVC pipe, a flower pot on its side, or a plastic plant. A fish with nowhere to hide stays stressed, and stress is what lets the parasites win.
• No substrate, no decor, no live plants. Bare glass shows uneaten food and waste, siphons clean in a minute, and gives ich cysts nowhere to hide. Medications that would kill plants and stain gravel do neither.
• Its own net, bucket and siphon, never shared with the main tank.

Set it up the day before the fish arrive: sponge moved over from the main tank, water from the main tank rather than fresh tap water so the chemistry matches, heater on and checked.

The four weeks for new fish

Week one: watch. Feed lightly, test ammonia every other day, and look at the fish for two minutes morning and evening. Flashing against the pipe, clamped fins, fast breathing, white spots, a gold dusting, stringy white waste, a ragged fin edge: any of these is the reason the tank exists. Most problems show in the first ten days.

Week two and three: treat if needed, or keep watching. A fish that shows a problem gets treated here, at full dose, with the ich guide or the fin rot guide as the plan, and the clock restarts when it is clear. A fish that shows nothing keeps eating and being watched. Many keepers run a routine deworming in week two whatever the fish looks like, because internal parasites show nothing until they have won.

Week four: move. Three to four weeks with no symptoms, eating well and active, and the fish goes into the main tank, acclimated slowly to the new water. The sponge goes back into the main filter to stay alive, the quarantine tank is drained and dried, and the bucket and net go back in the cupboard.

Water changes during quarantine: 25 % every two or three days, more if ammonia reads anything, always from the main tank or from conditioned tap water at the right temperature.

In the app — add the quarantine tank as a tank of its own, with the arrivals as its inhabitants: it gets its own test schedule and journal, and every reading and observation lands against the right fish. When they graduate, move them to the main tank’s inhabitants and the history comes with them.

Set up the quarantine tank →

Running it as a hospital tank

The same tank, the same day, for a fish that is sick in the main tank.

Move the fish, not the tank. Fill the hospital tank with water from the main tank so the fish feels no change, bring the sponge over, set the heater to the main tank’s temperature, and net the fish gently. If the condition is one the whole tank has, ich, say, treat the whole tank and skip the hospital; it is for the one fish that needs a dose the others should not get.

Dose for the actual volume. Twenty litres is twenty litres. Measure the tank once and write it on the lid, because every medication label assumes you know it, and overdosing a small tank is the classic way to kill the patient. Remove the carbon from the filter if there is any.

Change 30 to 50 % of the water daily, re-dosing the medication for the water you replace. The bare tank makes this a two-minute job, and clean water is half of every treatment.

Feed lightly, watch closely, and keep the lights low. A sick fish wants dim, quiet water and a place to hide.

After the course, wait. Three days past the last symptom before the fish goes home, and a final water test in the main tank first, because the reading that made the fish sick is often still there.

In the app — run a checkup with a photo and the readings, and Add plan to my schedule turns the treatment into dated tasks on the hospital tank: the daily water changes, the re-dose days, the day the course ends. The follow-up check-in asks whether the patient is recovering and closes the case in the journal when it does.

Run a checkup on the patient →

Common questions

How long should I quarantine new fish?

Three to four weeks, counted from the last day any fish in the quarantine tank showed a symptom. Most problems appear within ten days, but ich and velvet can hide for two weeks in cool water, and internal parasites longer. Shrimp and snails carry fewer fish diseases but can carry planaria, hydra and pest snails, so a fortnight in a jar with a sponge filter is worth it for them too.

Does a quarantine tank need to be cycled?

Yes, and the trick is to never let it be uncycled: keep its sponge filter running inside the main tank’s filter between uses, so the bacteria are already there the day you fill it. A quarantine tank started with a new sponge is an uncycled tank with a stressed fish in it, and you will be doing daily water changes against ammonia as well as watching for disease.

Can I use a bucket or a plastic box as a quarantine tank?

Yes. A 20-litre food-grade plastic box with a lid, a sponge filter and a heater is a perfectly good quarantine or hospital tank, and it stacks in a cupboard when empty. What matters is the filter, the heater, the hiding place and the lid; the material of the walls does not. Avoid anything that has held detergent or paint.

Should I medicate new fish as a precaution?

Not with antibiotics, which breed resistance and do nothing for the parasites that actually arrive in bags. Many experienced keepers do run a routine dewormer during quarantine, because internal worms show no sign until the fish is thin, and a praziquantel-based treatment is gentle. Beyond that, treat what you see. The point of the tank is that you get to see it.