A quarantine tank — also called a hospital tank — is the single most useful piece of equipment an aquarist can own, and the one most often skipped. It is where new fish wait before they join your display tank, and where a sick fish goes so that treatment does not damage everything else you keep.
Why treat in a separate tank
Antibacterials do not distinguish between the bacteria causing a problem and the nitrifying bacteria living in your filter media. Those nitrifying colonies are what convert ammonia to nitrite and nitrite to nitrate. Dosing a display tank can knock them back far enough that ammonia climbs while the fish is already weakened — a second, larger problem layered on the first. A separate tank keeps the medication away from your biological filter, away from invertebrates that tolerate it poorly, and away from substrate and decor that absorb and slowly release it.
It also lets you see what you are doing. In a bare tank you can count respiration, watch appetite, see faeces, and spot changes on the body that are invisible against gravel and plants.
What the tank needs
- Volume. Ten to twenty gallons suits most community fish. The tank should be large enough that water chemistry does not swing wildly, and small enough that you can change a large share of the water quickly and cheaply.
- A bare bottom. No substrate. It is easier to keep clean, easier to see waste, and there is nothing to bind medication.
- Filtration without carbon. A sponge filter is ideal. Activated carbon strips medication out of the water, so remove carbon cartridges before dosing and add fresh carbon afterwards when you want it gone.
- A heater and a thermometer. Stable temperature matters more than a particular number; match the display tank unless a vet advises otherwise.
- Cover and shade. A lid, and low light. Sick fish settle faster in dim conditions, and several antibacterials degrade in bright light.
- Somewhere to hide. A clean piece of PVC pipe or a plastic plant. Nothing porous.
Getting the filter ready before you need it
The common mistake is to set the tank up on the day of the emergency, when the filter holds no bacteria at all. Instead, keep a sponge filter running permanently in your display tank or sump. It seeds itself, and on the day you need it you move the sponge across and the quarantine tank is biologically active from the first hour.
If you do have to start cold, plan on daily water changes and test ammonia and nitrite every day. A filter that has not cycled will not protect the fish, so you become the filter.
Moving the fish
Fill the quarantine tank with water from the display tank so that chemistry matches, then top up with fresh dechlorinated water at the same temperature. Net the fish gently, or better, guide it into a container and move it in water rather than in air. Dim the lights and leave it alone for several hours before you do anything else.
Observation
Write down what you see each day: appetite, respiration rate, position in the tank, the appearance of fins, eyes, gills and skin, and the results of your water tests. A written record is what lets a veterinarian help you, and it is what tells you whether a treatment is working or whether the problem was never bacterial in the first place. Many symptoms aquarists treat as infection are water quality, parasites, or injury.
During treatment
Follow the label on the product you are using, complete the course, and keep testing the water. Antibacterials can suppress the filter even in a quarantine tank, so ammonia and nitrite need watching throughout. Feed lightly; uneaten food fouls a small volume fast.
Afterwards
When the course is finished, run fresh activated carbon to strip residual medication, then do a series of water changes. Give the fish a period of observation in clean water before it goes back to the display tank — a week is a reasonable minimum. Break the tank down, clean it without soap, dry it, and store it ready for next time.
Quarantining new arrivals, not just sick fish
Most disease enters a display tank on the back of a healthy-looking new fish. Shop tanks are often shared systems, and an animal that has just been bagged, transported and re-acclimated is immunologically at its worst. Two to four weeks in quarantine gives anything incubating a chance to appear while it is still contained, and gives the fish a quiet period to feed and recover from transport.
Do not medicate prophylactically during that period. Treating a healthy animal "just in case" exposes bacteria to a selective pressure for no benefit, and it removes your ability to tell whether a symptom that appears later is new. Quarantine is an observation period first.
Acclimation without shocking the fish
Float the sealed bag to equalise temperature, then add small volumes of tank water to the bag over twenty to thirty minutes so that pH and hardness shift gradually rather than in one step. Net the fish into the tank and discard the transport water rather than pouring it in. Keep lights off for the rest of the day.
A practical equipment checklist
- Tank, lid and stand that can stay set up, or stored flat and clean
- Sponge filter plus a spare sponge kept running in the display system
- Adjustable heater and an independent thermometer
- Air pump, airline and check valve
- Liquid test kit for ammonia, nitrite, nitrate and pH
- Dechlorinator, siphon, two buckets reserved for aquarium use only
- Fresh activated carbon, sealed, for post-treatment removal
- A notebook, or a note on your phone, for the daily record
What a useful daily record looks like
Date and time; ammonia, nitrite, nitrate, pH and temperature; what you fed and whether it was eaten; respiration rate at rest; position in the tank; and a one-line description of any change to fins, eyes, skin, gills or behaviour. Photograph anything visible, in the same light each day. Three days of that record tells a veterinarian more than a paragraph of recollection, and it is the only reliable way to know whether something is improving.
Common mistakes
- Setting the tank up on the day of the emergency. The filter is sterile and the ammonia produced by a stressed fish has nowhere to go.
- Leaving carbon in during treatment. It removes the medication you just added.
- Stopping early. The fish looking better is not the same as the course being finished, and the survivors of a partial course are the least susceptible bacteria.
- Combining treatments. Stacking products without veterinary direction makes it impossible to attribute either benefit or harm, and some combinations are antagonistic.
- Returning the fish immediately. Give it clean water and a period of normal feeding before it rejoins the display tank.
When more than one fish is affected
If several fish show the same signs at once, the environment is the more likely explanation, not a coincidental simultaneous infection. Test the water first. A heater that has failed, a filter that has stopped, an overdue water change, a dead fish decomposing behind the rockwork, or a dosing error will produce a tank-wide picture that no antibacterial will fix.
Sources
- US FDA, Center for Veterinary Medicine — unapproved animal drugs
- AVMA — antimicrobial use and antimicrobial resistance
- US EPA — nutrient pollution (nitrogen cycle background)
For aquarium and avian use only. This page is general husbandry information, not veterinary advice, and not a dosing instruction. It is not for human use. For a sick animal, consult an aquatic or avian veterinarian who can identify the organism and advise on the correct product, dose and duration.
Last reviewed: September 2026. We re-check this page each quarter and whenever a product label or policy changes.