Hospital tanks: why you need one and how to set it up

Why a hospital tank?

When a fish is sick, treating it in the main tank means dosing everyone — including inverts and sensitive species — and killing beneficial bacteria if you use meds that hit the filter. A hospital tank (or quarantine tank) lets you isolate the patient, treat only that tank, and avoid spreading disease. The same setup doubles as a quarantine tank for new fish: you observe them for a few weeks before adding them to the main tank, so you don’t introduce ich, velvet, or parasites.

What you need

• Tank: 5–10 gallons is enough for most single-fish treatment or small quarantine. Bare bottom is easiest to keep clean and avoids hiding meds in substrate.
• Heater: Match the temp to what the fish needs; stability matters.
• Filter: A simple sponge filter or small internal is fine. No carbon if you’re using medication (carbon removes many meds). If the tank isn’t running year-round, keep a sponge in your main tank filter so you can move it to the hospital tank and have instant cycled media.
• Lid: Reduces evaporation and jump risk.
• Optional: A hide (e.g. PVC or a small pot) so the fish can feel secure; no need for decor that traps waste.

Keeping it cycled

If the hospital tank sits empty most of the time, the filter won’t stay cycled. Options: (1) Run a spare sponge in your main tank and move it to the hospital tank when needed. (2) Keep a small permanent “quarantine” tank running with a couple of hardy fish or use it only when needed and do frequent water changes. (3) Use bottled bacteria and test; be ready for water changes if the cycle isn’t fully established. You want zero ammonia and nitrite when a sick or new fish is in there.

How to use it

For sick fish: Net the fish, add to the hospital tank (match temp and drip-acclimate if possible), then treat according to the illness (see our top 10 fish illnesses and ich guide). Do water changes as needed; some meds require partial changes between doses. When the fish is healthy and meds are done, move it back to the main tank carefully.

For new fish: Acclimate them to the quarantine tank and leave them there for 2–4 weeks. Feed lightly, observe for spots, odd behaviour, or weight loss. If all is well, acclimate them to the main tank. If something shows up, treat in the quarantine tank so the main tank stays clean.

Log which fish are in which tank and when you last dosed (e.g. in App-aquatic) so you don’t lose track. A hospital tank is one of the best investments a serious hobbyist can make.

Quick takeaways

• Hospital tank = isolate sick fish or quarantine new fish; protects the main tank and lets you treat with the right meds.
• Simple setup: small tank, heater, sponge or small filter (no carbon when medicating), bare bottom, lid.
• Keep media cycled (e.g. spare sponge in main tank) or be ready for extra water changes.