How to Set Up a Hospital Tank for Fish Antibacterials

A hospital tank is a small, simple aquarium used only for treating sick fish. It makes antibacterial doses accurate, keeps the treatment away from your main tank's filter, plants, and invertebrates, and lets you watch the patient closely.

Last updated November 3, 2026. Written and reviewed by the Fish-Mox Team.

What you need

• A tank of 10 to 20 gallons for most community fish, larger for big cichlids or goldfish
• A heater and a thermometer
• A sponge filter, ideally one that has been running in your main tank so it already carries beneficial bacteria
• An air pump and air stone, because treated water holds less oxygen
• A lid, since stressed fish jump
• A water test kit for ammonia and nitrite
• A clean bucket and a measuring cup used only for this tank

Leave out gravel and decorations, or use one smooth hiding place. A bare bottom is easy to clean and doesn't absorb treatments.

Measure the real water volume

Fish antibacterials are dosed by gallons, so a good estimate matters. For a rectangular tank, multiply length by width by water depth in inches, then divide by 231 to get gallons. A tank 20 inches long, 10 inches wide, with 11 inches of water holds about 9.5 gallons. Subtract a little for anything inside the tank. Write the number on a piece of tape on the glass so you don't recalculate every day.

Set it up

1. Fill with water from the main tank where possible, topped up with dechlorinated water at the same temperature.
2. Move the seeded sponge filter from the main tank and start the air stone.
3. Match the temperature to the main tank within a degree or two.
4. Move the fish gently, with as little time in the net as possible.

Before the first dose

• Don't put activated carbon, resins, or chemical media in the hospital tank's filter. They pull antibacterials out of the water.
• Don't run a UV sterilizer during treatment.
• Dim the lights. Some antibacterials, including doxycycline and ciprofloxacin, break down faster under strong light.

The daily routine

1. Test ammonia and nitrite. Treatment can slow even a seeded filter.
2. Change part of the water, 25 to 50 percent, with dechlorinated, temperature-matched water.
3. Add the full dose for the tank's volume, dissolved in a cup of tank water first.
4. Watch the fish for breathing, appetite, and changes in sores or fins, and note them down.

Every Fix product has a dosing chart and a dose helper on its page. For example, a 10 gallon hospital tank needs one 250 mg Fish Mox capsule a day, or one 250 mg Fish Zole tablet for internal flagellates.

After treatment

• Finish the full course on the label before moving the fish back.
• Do a large water change and run fresh carbon for a day or two to clear what's left.
• Watch the fish for a few more days, then return it to the main tank once it eats normally.
• Never pour treated water into the main tank. Dispose of it away from drains where you can, following local guidance.

Between uses

Empty the tank, clean it, and let everything dry completely. Keep the sponge filter running in your main tank so it stays seeded for next time. A hospital tank that's ready to go is also a quarantine tank: holding new fish in it for 2 to 4 weeks prevents many of the problems antibacterials are needed for.

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