Setting up a hospital tank

A good deal of this site tells you to treat a fish in a separate tank. This is the page about that tank — what it has to be, what it does not need, and how to run one that is not cycled, which most of them are not.

It is worth starting with a distinction that causes confusion, because the two jobs share the same equipment and are otherwise unalike.

Quarantine and hospital are different jobs

What differs	Quarantine tank	Hospital tank
The fish	Well, newly acquired	Sick or injured
Duration	At least four weeks	Days to a few weeks
Purpose	Observation, and protecting the display	Treatment, and precise dosing
Filtration	Cycled sponge, kept running	Cycled if possible, usually compromised
Medication	None, unless something appears	The whole point of it
Feeding	Normal	Light, or none
Water changes	Routine	Daily, driven by test readings

The same box, the same sponge filter, the same heater and lid. But a quarantine tank is a quiet room and a hospital tank is a procedure, and running one like the other is how people end up with an ammonia problem on top of a sick fish. The quarantine guide covers the first job; this page covers the second.

First: should the fish be moved at all?

Moving a sick fish costs something, so the destination has to be worth it. The medications page sets out the full argument; the short version:

Treat the display tank when the organism lives in it as well as on the fish. White spot is the clearest case — the parasite spends part of its life cycle in the substrate and the water column, so treating one fish in isolation and returning it to an untreated tank achieves nothing. The same goes for anything affecting several fish at once.

Use a hospital tank for a localised problem on one fish: an ulcer, a fungal patch, an injury, or anything needing a medication you would rather not put through your display tank’s filter.

Do not move the fish at all when it is too weak to withstand being caught, when it is a pond fish in winter torpor, or when catching it would mean a long pursuit around a decorated tank. The handling guide covers how to make the catch short, which is the variable that decides whether this is a small event or a large one.

The tank

Size: bigger than the word “temporary” suggests. For an adult fancy goldfish, 40 to 60 litres is a sensible floor; for a single-tail, more. The instinct is a small tub on the grounds that the fish is only there for a week, and the reasons that fails are all the ordinary ones made worse: a small volume swings in temperature, fouls fast without a cycle, holds less oxygen, and multiplies any error in a dose. A large opaque storage crate with a lid is a perfectly respectable hospital tank and costs almost nothing.

Bare bottom. No gravel, no sand. Substrate absorbs medication unpredictably — copper in particular binds to it and leaches back — it cannot be cleaned properly afterwards, and it hides exactly the waste you want to be looking at. A bare floor also lets you see the faeces, which on a fish with a gut problem is diagnostic information.

No porous decor. Rock, wood and ornaments all take up medication and none of them can be sterilised easily at home.

Somewhere to hide, all the same. A length of clean plastic pipe or a plastic plant, wiped down between uses. A brightly lit empty box is a stressor in its own right, and a sick fish that can get out of the light does better than one that cannot. This is the detail most often left out of the bare-tank advice, and it matters.

A lid. A stressed or recently netted fish jumps, and a hospital tank is usually shallower than the display.

Dim light or none. A hospital tank does not need lighting at all. Ambient room light is enough to observe by, and dark is calming.

Filtration, and the spare sponge that makes this easy

Use an air-driven sponge filter, and for three specific reasons rather than out of habit:

• No carbon. A cartridge filter often contains activated carbon, sometimes hidden inside the floss, and carbon strips medication out of the water as fast as you add it. The filter media guide covers which cartridge holds what.
• No impeller. A weak fish, or one with long fins, can be pulled against a powerhead intake. A sponge cannot do that.
• Gentle flow. A sick goldfish does not want to swim against a current.

And then the practice that turns this whole page from a project into a ten-minute job:

What you must not do is take the display tank’s only filter. That solves the hospital tank by creating an uncycled display tank containing all your other fish.

Heating and aeration

A heater with a guard, set to match the display tank to begin with. Goldfish rest against warm surfaces and can burn themselves on an unguarded element, and a fish that is already unwell is likelier to be sitting still against something.

Temperature matters here beyond comfort: most treatments assume a metabolism that is running, and the fish’s own immune response follows the water. The temperature guide sets out the range and the heaters page the hardware.

Raising the temperature is a real protocol and a limited one. For some parasites, warmer water speeds the life cycle so the vulnerable stage arrives sooner. It is used routinely with tropical fish. With goldfish it is a smaller lever, because they are cold-water animals and because warm water holds less oxygen at the moment the fish needs more. If you do it: go gradually, a degree or two a day, stay within the comfortable range, and add aeration before you add heat. Check the product’s label for an upper temperature limit first, because some treatments, formalin among them, have to be dosed lower as the water warms.

Aeration, always, for the whole course. An air stone, or the sponge filter’s own outflow breaking the surface. Several treatments reduce available oxygen directly, and the aeration guide explains the margin a goldfish has, which is less than most aquarium fish.

A hospital tank

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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 water

Fill it from the display tank. A sick fish is already being caught and moved; filling the hospital tank with fresh tap water means it meets a change of chemistry in the same minute. Display water makes the move one variable instead of two.

Take the water before you take the fish, so the tank is at temperature and settled when the fish arrives.

Top up and change with conditioned tap water, matched for temperature, from then on. The source water page covers what is actually in it, and the conditioner requirement is the same as for any other water change.

Running one that is not cycled

This is the part that gets skipped, and it is most of the work.

A hospital tank often has no established bacteria, and even a seeded sponge is likely to be damaged by an antibacterial course. So you cannot rely on biology to deal with ammonia. You deal with it directly:

• Test ammonia and nitrite every day, without exception. This is not optional in an uncycled tank with a fish in it.
• Change 25 to 50 percent whenever either reads above zero, and at least 50 percent at once if ammonia is high, with conditioned, temperature-matched water.
• Re-dose the medication for the fraction removed if the product’s instructions say to — read that specific line, because products differ.
• Feed lightly, or not at all. A fish under treatment often is not eating properly anyway, and uneaten food in a tank you cannot clean normally is the fastest route to an ammonia reading.
• Siphon waste off the bare floor daily. Thirty seconds, and it removes most of the load before it becomes ammonia.

An ammonia-binding conditioner can serve as a bridge — it converts ammonia to a form the fish is not harmed by while leaving it available to whatever bacteria are present. It holds the ammonia for only a day or two, so it has to be re-dosed. Be aware that some test kits read high with the conditioner in the water, and that Nessler-type kits also read falsely high when formalin is present, so use a salicylate-based kit if you are treating with formalin; a high reading from the wrong kit is the chemistry rather than a crisis.

It is not only for medication

Worth saying, because “hospital tank” suggests a course of treatment and several of its best uses involve nothing going into the water at all.

Clean water and warmth on their own. A fish recovering from an injury, a torn fin, or a scrape from a decoration usually needs pristine water and time rather than a bottle. A hospital tank with daily water changes delivers water cleaner than any display tank can sustain, and that is often the whole treatment. It is worth being clear about when that is worth the move, though: first aid for an injured goldfish argues that the default for a minor injury is to leave the fish where it is and improve the main tank, and reserves this for damage serious enough that daily changes are the point.

Salt, and nothing else. A great many hospital-tank episodes are a salt regime rather than a medication, which the salt guide covers — dosed against the tank’s real volume, which is far easier to know in a bare tank than in a display full of gravel and rock.

A fish being harassed. Separation stops the harm immediately. Note the counterintuitive version: if one fish is doing the harassing, it is often better to move the aggressor for a week and then return it, which resets the arrangement, than to move the victim and return it to the same situation. Chasing and bullying covers working out whether separation is the right answer at all, since most reported bullying turns out to be spawning, food competition or crowding.

A fish that cannot compete for food. A slow, elderly or recently unwell goldfish in a group of fast eaters may simply not be getting enough. A week alone with unhurried feeding settles whether that is what is happening, and caring for an old goldfish covers the feeding arrangements that keep it from happening again.

Observation. In a bare, well-lit, uncluttered tank you can actually see a fish — its breathing rate, the way it holds its fins, what it produces, whether it eats. A great deal of what a hospital tank is for is simply being able to look properly, against the healthy baseline.

A rest day under supervision. Fasting a fish with a gut or buoyancy problem is easier to do properly when it is not in a tank where five other fish are being fed.

Two fish at once

Generally one fish, one hospital tank, for reasons that are mostly about dosing and observation: you cannot tell which fish produced what, and a medication that suits one may not suit the other.

The exceptions are real, though. A bonded pair that panics apart, a whole small group affected by the same thing, or fry being grown on all make sense together. If you do put more than one in, size the tank for the combined fish rather than for the idea that it is temporary, and expect the ammonia management to be correspondingly more work.

If you have nothing and need one tonight

A fish will not wait for an online order. The minimum viable version, in descending order of how much you have:

A clean bucket or storage box, never used for detergent, filled with water from the display tank. Somewhere to hide. Something over the top so the fish cannot jump out — a towel across most of it works.

An air stone if you have one, and if not, a partial water change morning and evening to keep oxygen up. Surface movement is what you are after.

A heater if the room is cold, or the bucket standing somewhere the temperature does not swing, which matters more than the exact number.

Water changes of 50 to 100 percent daily, with conditioned water matched for temperature. Without any filtration at all, water changes are the entire life-support system, and in a bucket they are quick.

No food, or barely any, for the first day or two.

That will hold a fish safely for several days, which is long enough to buy a sponge filter or speak to a vet. It is not a substitute for the proper setup, and it is a great deal better than leaving a fish where it is because the proper setup does not exist yet.

The daily routine

Every day	Why
Test ammonia and nitrite	The tank cannot be assumed to process them
Look at the fish properly, for a minute	Improvement and deterioration both show first in behaviour
Siphon the floor	Removes waste before it becomes ammonia
Water change on the reading	Not on a schedule
Dose as the label directs	Including any top-up for water removed
Write down what you did	Date, dose, readings, what changed

The written record is worth more than it looks. It is the thing a vet will ask for, and it is what stops the common failure where nobody can remember whether the second dose went in.

Ending it

Finish the course, not the visible sign. A fish that looks better on day three of a seven-day course has parasites that have been knocked back rather than eliminated.

Wait a few days after the course with the fish eating and behaving normally, checked against the healthy baseline.

Acclimate back. The two tanks have drifted apart — the hospital tank may be warmer, may have salt in it, may sit at a different pH after a week of daily changes. Add display water to the hospital tank gradually over half an hour, then move the fish.

Do not tip hospital water into the display. It carries medication residue.

Afterwards

The sponge does not go back into the display filter if it has been through medication. It has absorbed the treatment along with everything else, and returning it carries the residue into the system you were protecting. Keep a dedicated hospital sponge, rinse it thoroughly in dechlorinated water, and either re-seed it in the display long afterwards or treat it as a consumable.

Clean the tank down with hot water and a scrub, no detergent. If you are worried about an infectious disease, many keepers disinfect it with a dilute bleach solution (this site gives no strength because none is sourced) followed by a very thorough rinse, a dose of dechlorinator in the rinse water to neutralise any trace, and several days of drying — bleach breaks down and drying finishes the job, but the rinse has to be genuine.

Store it dry and together, tank, sponge, air pump, heater, siphon, lid, in one place. The value of this whole setup is that it is ready.

What to own, and why before rather than during

The complete kit:

• A 40–60 litre tank, or a lidded storage crate
• An air pump, airline and a sponge filter — plus a spare sponge living in the display tank
• An adjustable heater with a guard
• A thermometer
• A siphon and a bucket kept for this
• A length of clean pipe for the fish to sit beside
• A liquid test kit for ammonia and nitrite

It is not much, and assembling it on the evening a fish develops an ulcer means an hour lost at the point where hours matter. Assembled in advance and stored in a cupboard, it turns a crisis into a procedure.

The mistakes

Too small, because it is temporary. It is temporary and the fish is unwell; those argue in opposite directions.

Substrate left in, which absorbs the treatment and cannot be cleaned.

A cartridge filter with carbon, which quietly removes the medication and produces the conclusion that the treatment did not work.

Taking the display tank’s filter, which converts one problem into two.

No lid, in a shallow tank holding a stressed fish.

Nowhere to hide, which is a stressor that costs nothing to fix.

Running it like a display tank — weekly changes, normal feeding, no testing. That is the one that does the most damage, and it is the subject of half this page.