How to quarantine a new goldfish

Read enough goldfish advice and you will be told to quarantine new arrivals perhaps a dozen times, usually in a single clause at the end of a paragraph, and rarely with any instructions attached. This page is the instructions. None of it is expensive and none of it is difficult, but the order matters: a quarantine tank set up on the day the fish comes home is a bucket of uncycled water with a stressed animal in it, which is worse than no quarantine at all.

What you are actually guarding against

Shop and breeder systems are usually shared water, which means one tank’s problem is every tank’s problem. A fish that looks healthy under shop lights may be carrying ich, flukes or a bacterial infection that a week of netting, bagging and transport is about to turn into something visible. Adding that fish straight to an established tank does not risk one animal; it risks every animal in the tank, plus the filter, plus whatever you end up pouring into the water to fix it.

There is a second, quieter reason. A new goldfish in a busy display tank is almost impossible to assess. You cannot easily tell whether it ate, whether its faeces look normal, whether it is breathing faster than the others, or whether it spends its nights on the substrate. In a plain tank with one fish in it, all of that is obvious within a day.

The tank

Plain is the whole design brief.

Volume. Around 40 to 75 litres (10 to 20 US gallons) is enough for one goldfish for a few weeks. It is not a home and the fish is not growing into it; it is a place to be watched. Bigger is still better, because a small volume fouls quickly.

A bare bottom. No gravel, no sand. Debris and uneaten food stay in plain sight and siphon out in seconds, and there is nowhere for parasite eggs to settle out of reach.

A seeded sponge filter. This is the part that has to be started in advance. A sponge filter run in your main tank for three or four weeks arrives in the quarantine tank already colonised, so the fish never sits in water that is still cycling. Sponge filters suit the job for other reasons too: they cannot injure fins, they run on an air pump, and they are cheap enough to own several.

A lid, a thermometer and somewhere to hide. A frightened goldfish jumps. A length of PVC pipe or an upturned plastic pot gives it somewhere to feel unwatched, and unlike decor it can be scrubbed and dried afterwards without regret.

Its own net and siphon. Equipment moves disease between tanks faster than anything else does. Either buy a second set, which is inexpensive, or dry the shared ones completely between uses, since drying kills most of what you are avoiding.

A hospital tank

1. 1
2. 2
3. 3
4. 4
5. 5
6. 6

1. Bare bottom. Nothing to absorb the medication, and nothing to hide the droppings, which on a sick fish are information.
2. Air-driven sponge filter, colonised in the display tank beforehand: no carbon, no impeller, gentle flow.
3. A heater inside a guard, set to match the display tank, so the fish cannot touch it.
4. Somewhere to hide, such as a length of clean plastic pipe. A bright empty box is a stressor of its own.
5. A lid. A stressed or recently netted fish jumps. No lamp is needed; room light is enough.
6. Aeration for the whole course, here the sponge filter’s own lift tube breaking the surface.

The first hour

Match the temperature before anything else. Float the sealed bag in the quarantine tank for fifteen to twenty minutes, then add a little tank water to the bag every few minutes for another twenty, and net the fish across. Leave the shop water in the bag and pour it down the sink rather than into the tank. The tank setup guide covers the same sequence for a first fish.

Then do nothing. Lights off or dim for the rest of the day, no feeding until the next morning, no faces at the glass. A fish that has spent the day in a bag has enough to recover from.

What to watch for

Look at the fish twice a day for the first week and once a day after that. You are watching for the things the health section describes one by one, and the useful habit is to check the same short list every time.

• Breathing. Count the gill beats. Faster than the same fish was yesterday, or obviously laboured, is the earliest signal you will get.
• Fins. Held away from the body and moving is normal; clamped tight against the flanks is not.
• Flanks and fins under a torch. Grains of salt, cotton-like tufts, red streaks, cloudy patches, missing scales.
• Behaviour. Rubbing against the glass or the filter, hanging at the surface, sitting on the bottom for long stretches.
• Appetite and faeces. Refusing food after the first day or two is meaningful, and long, pale, trailing faeces suggest the gut is not working properly.

Test the water every two or three days. A small bare tank with a fish in it and a filter that is only recently seeded can produce an ammonia reading quickly, and at this volume a water change fixes it in minutes.

What the four weeks actually look like

The attention is front-loaded, which is worth knowing before you start, because the common failure is to watch hard for four days and then forget.

When	What you do	What you are looking for
Day 1	Acclimatise, lights low, no food	Nothing. The fish is recovering from the journey.
Days 2–7	Look twice a day, feed lightly, test every other day	Much of what appears, appears here: clamped fins, flashing, refusing food, faster breathing, the first spots.
Week 2	Look daily, feed normally, test twice	The second wave of a parasite cycle. A fish that was fine in week one and is flashing in week two is telling you something.
Weeks 3–4, and on to six if the water is cool	Look daily, test weekly, change water weekly	Confirmation. Mostly you are turning “probably fine” into “fine”.
The day it moves	Match temperature, net across, leave the water behind	Whether the main tank settles over the following week.

Many problems declare themselves in the first week or two. The remaining weeks are not padding, though: they are what covers the second and third rounds of a parasite cycle, which runs slower at goldfish temperatures than the figures printed on most medication boxes assume.

More than one fish

Straightforward once the principle is clear: quarantine is about keeping water separate, not about keeping fish alone.

Two fish from the same shop tank go in together. They have shared water for weeks already, so separating them now prevents nothing and halves the space each one has. Goldfish are also calmer with company, which makes a stressful few weeks easier.

Two fish from different shops, or different systems in the same shop, do not. Putting them together is precisely the mixing that quarantine exists to prevent, and it means a problem in either one becomes a problem in both. Two containers for a month is tedious. It is considerably less tedious than the alternative.

If you are stocking a tank from scratch, the least painful route is to buy the whole group at once from one healthy system and quarantine them together — and then not add fish again for a long time. Every subsequent addition is another four weeks and another risk, which is one more argument for deciding the final stocking number at the start rather than drifting toward it.

Plants, snails and second-hand equipment

The gap in almost everybody’s biosecurity, including people who quarantine fish diligently.

Plants are the big one. Shop plants are usually grown or held in tanks that contain fish, and they arrive carrying snails and snail eggs, hydra, planaria and sometimes the free-swimming stage of ich. The fix is time or a dip:

• Time. Hold the plants in a bucket or spare tank of dechlorinated water, with a light, for four to six weeks. Anything that needs a fish to complete its life cycle runs out of hosts; snails, hydra and planaria do not.
• A dip. Faster and harder on the plants. Some keepers use a dilute bleach or potassium permanganate solution, briefly, then a thorough rinse and a dechlorinator soak; no sourced strength or contact time is given here, and delicate species do not always survive it.

The plants guide covers what survives a goldfish in the first place, which narrows the list considerably.

Snails deserve the same treatment and rarely get it, because they usually arrive as stowaways rather than purchases. The snails and hitchhikers page covers which ones matter and which are simply part of the tank.

Second-hand tanks, decor and equipment carry whatever was in their last tank, and a used filter is the worst offender because it is designed to concentrate exactly that. Scrub hard items, disinfect them (dilute bleach is the usual choice; no sourced strength is given here), rinse until there is no smell, and then dry them completely — drying kills most parasites and many bacteria, though not reliably Mycobacterium, and it is the cheapest disinfectant there is. The decorations page covers what is safe to put in a goldfish tank at all.

The argument for treating preventatively

Worth setting out properly rather than dismissing, because it is held by serious keepers and you will meet it.

The case for it. Common parasites — flukes in particular — are often present at low levels without visible signs, and a fish can carry them for months before anything shows. The quarantine tank is the one place where treating is cheap, contained and does not risk the display tank’s filter. So, the argument goes, run a standard course during quarantine and you start every fish clean.

Why this site does not recommend it for a general keeper. Four reasons, none of them decisive on its own:

• It means dosing a fish you have no evidence is unwell, and every medication has a cost to the animal.
• Many treatments harm filter bacteria, which is a problem in a tank whose filter you seeded specifically for this.
• Routine treatment of healthy animals is how resistance develops, and the goldfish hobby has a small number of effective compounds to lose.
• A fish under treatment is harder to read. If something appears in week three, you now cannot tell whether it is a new problem or a reaction to the medication.

If you do choose to treat, treat deliberately rather than by habit: one compound at a time, at the label dose, with extra aeration, and using medications safely read first. What nobody should do is combine several products because a forum post listed them together.

If something does appear

Two things are true at once: much of what shows up in quarantine is stress and poor shop water acting on parasites and bacteria the fish already carries, and the fastest thing you can do for a sick goldfish is nearly always clean, stable water. Change water, hold the temperature steady, stop feeding heavily, and look properly before reaching for a bottle.

Where a treatment is warranted, treat in the quarantine tank and nowhere else, which is the other half of why the tank exists. Follow the product’s own instructions rather than a figure from a forum, and note that many treatments reduce dissolved oxygen, so extra aeration goes in alongside them. Aquarium salt has a real but narrow role here and is also often over-used in the hobby; it is not a general tonic, and it is a poor idea in a tank with live plants. If a fish is deteriorating, has stopped eating for several days, or shows swelling or raised scales, an aquatic veterinarian is the right call rather than a third bottle.

If the fish dies in quarantine

It happens, more often than anyone advertises, and there are a few things worth doing.

Leave the sponge filter out of your main tank. It has been sitting in the water the fish died in, and it is the one object most likely to carry something across. Strip the tank, discard the water, and clean and dry every part of it completely — the equipment, the pipe, the net, the siphon. If the cause was unexplained, scrub with 70 percent alcohol as well, because bleach alone is not enough against Mycobacterium, and wear gloves.

Think back over the days before. What the fish was doing at day three is usually more informative than what it looked like at the end, and why did my goldfish die works through the common patterns.

Go back to the shop. A fish that dies within a fortnight of purchase was often already in trouble when it was sold, and a reputable shop will want to know — both because most offer some guarantee and because it is information about their own system. This is not a complaint; it is a data point they need.

And do not conclude that you are bad at this. A fish that arrives after crowded transport and poor water may not survive whatever the new keeper does; the lifespan guide counts that among the reasons goldfish die early, and it is the one that gets blamed on the keeper unfairly. It is an argument for buying better next time, not for giving up.

Ending quarantine

If four weeks or more pass (six if the water was cool) with a fish that eats well, breathes calmly, holds its fins open and has produced nothing alarming, it is ready. Match the temperature between the two tanks, net the fish across, and leave the quarantine water behind. Then watch the main tank for a week or so, because introducing any fish shifts the pecking order and the bioload at the same time; check that your stocking numbers still work for the total, not just for the newcomer.

Dry the quarantine tank, the net and the siphon afterwards, but not the sponge. Return the sponge to the main tank only if the fish stayed well and was never medicated; otherwise discard it, or sterilise it and reseed it.

What quarantine does not do

The real limits, because a page that promises too much gets abandoned the first time it fails.

It protects your existing fish, not the new one. The new fish spends four weeks alone with whatever it brought. If it is carrying something, quarantine does not save it; it stops the rest of the tank from joining it.

It does not catch everything. A fish can be a carrier — infected, infectious and showing nothing — for far longer than four weeks. Some bacterial and viral infections only declare themselves under stress, which is to say, after the move into the main tank. Four weeks catches much of what matters; it does not catch all of it.

It does not fix a bad purchase. A fish bought from a system with dead fish in it is a bad idea that quarantine merely contains. The choosing guide is the step that does more for your odds than anything on this page.

And it does nothing about the tank you already have. If something appears in the main tank a month after a quarantined fish went in, the fish is not automatically the culprit — why did my goldfish die and goldfish diseases explained both start with the water, and so should you.

If you only have one tank

Most people reading this do not own a spare tank, and telling them to quarantine anyway is not advice, it is a shrug. The real ranking, best to worst:

1. A proper quarantine tank with a seeded sponge filter, for at least four weeks.
2. A food-grade tub or storage bin with the same seeded sponge and the same duration. Cheaper and usually roomier; you just have to lift the fish into a clear container to inspect it properly.
3. No isolation, but buy well and watch closely. Choose a shop where every tank on the system looks healthy, not only the one you are buying from, refuse a fish from a tank containing a dead or obviously sick one, and accept that you are taking a risk you cannot undo.

There is no useful fourth option. If a new fish is going straight into an established tank, at least know where it would go if something appeared, because the day you need a second container is the day you will not want to be shopping for one.