How to Set Up a Fish Hospital (Quarantine) Tank

By the PetRemedyUSA team · Updated 2026-09-30 · 5 min read

Quick answerA hospital tank is a small, bare, well-aerated tank used to treat or quarantine fish away from your main aquarium. Use a 10 to 20 gallon tank with a heater, a seasoned sponge filter, hiding places and no substrate or carbon, and keep water temperature and quality stable throughout treatment.

Why a hospital tank matters

Medicating your display tank can kill the beneficial bacteria in the filter, stain decorations, and expose healthy fish to drugs they do not need. A separate tank lets you dose precisely, observe the patient closely and protect the main system.

What you need

Tank: 10 to 20 gallons suits most community fish. Larger species need a larger container.
Filter: an air-driven sponge filter is ideal, ideally seeded in your main tank for a few weeks so it carries beneficial bacteria.
Heater and thermometer: stable temperature reduces stress.
Hiding places: PVC pipe sections or plastic plants let shy fish feel secure.
Lid: sick fish sometimes jump.
Test kit: for ammonia, nitrite, nitrate and pH.

What to leave out

Substrate: a bare bottom makes waste easy to see and remove.
Activated carbon: it absorbs medication and makes treatment ineffective.
Ornaments that trap debris: keep it simple and easy to clean.

Cycling the hospital tank

An uncycled tank can spike ammonia, which is dangerous to a sick fish. Seed the sponge filter in your established tank first, or add a bottle of beneficial bacteria. Test daily during use and be ready to change water if ammonia or nitrite rises.

Moving the fish

Match temperature and, as closely as practical, water parameters.
Use a net or a container to move the fish gently to limit stress.
Dim the lights for the first day.
Begin treatment according to the medication label.

During treatment

Test water each day and change water as the medication directions allow, replacing medication for the volume removed.
Feed lightly and remove uneaten food.
Keep a simple log of dates, doses and symptoms.

After treatment

Finish the full course, then run water changes and a small amount of carbon to remove leftover medication. Observe the fish for several days before returning it. Clean and dry the hospital tank so it is ready for the next use.

Tip: Keep the tank set up permanently with a dry sponge filter stored in a bag of tank water, so it is ready in an emergency.

Frequently asked questions

What size should a hospital tank be?

Ten to twenty gallons covers most community fish. Choose a size that lets the patient swim comfortably while keeping medication volumes manageable.

Do I need to cycle a hospital tank?

Yes. A cycled or seeded filter prevents ammonia spikes. Seeding a sponge filter in your main tank for a few weeks is the easiest method.

Why remove activated carbon when medicating?

Carbon adsorbs many medications, which lowers the effective dose in the water and can make treatment fail.

This guide is for general education and does not replace advice from a licensed aquatic or avian veterinarian.