A hospital tank is a small, separate aquarium used to treat sick fish. It keeps medication away from your display tank’s plants, invertebrates and filter bacteria, lets you dose accurately and makes it easier to watch the patient.
What you need
- A 10–20 gallon tank (larger for big fish), with a lid.
- A heater and thermometer set to the same temperature as the main tank.
- A sponge filter or air stone. Medications and warm water lower oxygen, so keep good surface movement.
- No gravel or substrate: a bare bottom is easier to keep clean.
- A simple hiding place, such as a clean pot or PVC pipe, to reduce stress.
- A water test kit for ammonia and nitrite.
Setting it up
- Fill the tank with water from your main aquarium, or dechlorinated tap water matched for temperature and pH.
- If you can, run the sponge filter in the main tank for a few weeks beforehand so it carries beneficial bacteria.
- Do not use activated carbon: it removes medication from the water.
- Move the sick fish gently, in a container rather than chasing it with a net where possible.
During treatment
- Follow the dosing and water-change directions on the medication label.
- Test ammonia and nitrite daily: medications can slow beneficial bacteria, and ammonia can build up quickly in a small tank.
- Feed lightly and remove uneaten food.
- Keep the lights dim and the area quiet.
After treatment
Return the fish only when it has finished the full course and looked healthy for several days. Then empty, clean and dry the hospital tank and equipment so it is ready for next time.
Not sure which medication to use? Read How to choose an aquarium antibiotic.
Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.