How to Set Up a Hospital Tank (Quarantine Tank) for Sick Fish

A hospital tank is a small, bare-bones aquarium used to treat sick fish away from your display tank — and it is the single best investment a fish keeper can make. Treating in a separate tank protects your main tank’s beneficial bacteria from antibiotics, lets you dose medication such as Fish Mox (amoxicillin) accurately in a known volume of water, and removes the stress of competition while a fish recovers. A basic 10-gallon setup costs little and takes under an hour to prepare.

What do you need for a hospital tank?

• A 10–20 gallon tank (bigger for large cichlids or goldfish)
• A simple sponge filter — gentle flow, no carbon, and it can be pre-seeded in your main tank’s filter so it arrives already cycled
• A heater and thermometer matched to your display tank’s temperature
• Bare bottom, no substrate — easier to clean and lets you see waste and uneaten food
• A hiding place such as a PVC elbow or smooth clay pot to reduce stress
• A dedicated net and bucket so you don’t carry pathogens back to the display tank

How do you set it up quickly when a fish is already sick?

1. Fill the tank with water from your display tank (a big water change on the main tank at the same time is a bonus), topping up with dechlorinated fresh water.
2. Move a seeded sponge filter or a handful of filter media from the main tank into the hospital filter — this gives you instant biological filtration.
3. Match the temperature before transferring the fish, then acclimate it gently.
4. Keep lighting dim and the tank partially covered — sick fish settle faster in low light.

How do you medicate in a hospital tank?

Dose for the hospital tank’s actual water volume, following the medication label — for most aquarium antibiotics in our fish range, typical label directions are one capsule or tablet per 10 gallons every 24 hours, with a partial water change between doses. Because there’s no carbon and a known volume, dosing is far more precise than in a decorated display tank. Watch ammonia daily with a test kit: antibiotics can slow even a seeded filter, and a 25% daily water change (re-dosing medication proportionally per the label) keeps the water safe. After treatment ends, run carbon for a day or two to pull out residual medication before returning the fish.

Should new fish be quarantined too?

Yes — the same tank doubles as a quarantine tank. Two to four weeks of observation for every new arrival catches most infections, parasites, and fin rot before they ever reach your display tank. Many experienced keepers consider quarantine non-negotiable; it is far easier to treat one new fish in a bare 10-gallon than an entire community tank.