Fish health · Treatment basics
How to Set Up a Hospital Tank for Sick Fish (Step by Step)
A hospital tank is the single most useful setup a fishkeeper can have — a place to treat a sick fish without dosing the whole display tank. Here's how to set one up.
Quick answer: A hospital tank is a bare, simple aquarium (often 10 to 20 gallons) with its own heater, gentle filter, and a place for the fish to hide. You move the sick fish there to treat it, which protects your main tank and lets medication work at the right dose without harming your filter or other fish. Set it up bare-bottom, keep the water warm and clean, and treat only there, never dose your whole display tank by default.
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Treat in the hospital tank, not your display tank
It is tempting to pour medication straight into your main aquarium, but this is often a mistake. Many antibiotics and treatments kill the beneficial bacteria in your filter, which can trigger a dangerous ammonia spike, and they expose healthy fish and invertebrates to drugs they don’t need. A hospital tank lets you treat the one fish that needs it, at the correct dose, without wrecking the balance of your display tank.[1]
What you need
A hospital tank is deliberately simple. The basics:
A bare tank, 10 to 20 gallons. Big enough to house your largest fish comfortably, small enough to dose accurately and change water easily. Bare-bottom (no gravel) makes cleaning simple and stops medication from being absorbed by substrate.
A heater. Sick fish need stable, species-appropriate warmth. A separate heater you keep with the hospital kit means you’re never scrambling.
A gentle filter, ideally a sponge filter. A sponge filter is safe, can’t trap a weak fish, and doesn’t contain carbon (carbon removes medication from the water, which you don’t want during treatment).[2]
Somewhere to hide. A simple plastic plant or a piece of PVC pipe lowers stress. Nothing porous that would soak up medication.
A lid, a thermometer, and dechlorinated water. The essentials that keep conditions stable.
Step by step
1. Fill with dechlorinated water. Use conditioner to remove chlorine. If you can, use some water from your established display tank to ease the move.
2. Add the heater, filter, and thermometer. Set the temperature to your fish’s normal range and let it stabilize before the fish goes in.
3. Move the sick fish gently. Net or cup it with care. A stressed, sick fish is fragile. Acclimate it to the hospital tank the way you’d any new water.
4. Observe before you medicate. Confirm what you’re actually treating. Not every problem is bacterial, and medicating blindly wastes time and stresses the fish. If you’re unsure, our fish-health guides can help you match the signs to the likely cause before you dose.
5. Treat as directed, in the hospital tank only. Follow the medication’s instructions for that tank’s volume. Keep the water clean with small changes, and re-dose only as the label directs.
Keeping it running
A bare hospital tank with no fish in it’ll not keep a cycled filter alive for long, because there’s no waste to feed the beneficial bacteria. Two common approaches: keep a sponge filter running in your display tank at all times, so it’s already colonized and ready to drop into the hospital tank the moment you need it; or plan to monitor water closely and do frequent small water changes during treatment, since the tank won’t be fully cycled. Either way, test the water and change it as needed. A sick fish in dirty water won’t recover.
When a hospital tank isn’t enough
A hospital tank is for isolating and treating, not for guessing. If your fish isn’t improving, if you’re treating repeatedly, or if you can’t identify the problem, that’s a signal to look more carefully at water quality and diagnosis rather than cycling through medications. The tank buys you a safe place to treat. It doesn’t replace correctly identifying what’s wrong.
Frequently asked questions about hospital tanks for fish
What size hospital tank do I need?
For most home aquariums, 10 to 20 gallons works well, big enough for your largest fish to be comfortable, small enough to dose and clean easily.
Why not just medicate my main tank?
Many medications kill the beneficial bacteria in your filter and can cause an ammonia spike, and they expose healthy fish and invertebrates to drugs they don’t need. Treating in a separate tank avoids both problems.
Do I need a filter with carbon?
No, avoid carbon during treatment, because it removes medication from the water. A simple sponge filter is ideal.
How do I keep a hospital tank cycled when it’s empty?
Run a spare sponge filter in your display tank so it’s always colonized and ready to move over, or plan on frequent small water changes and close monitoring during treatment.
How long does a fish stay in the hospital tank?
Long enough to complete the treatment and confirm recovery, often a couple of weeks. Do not rush a fish back into the display before it’s clearly better.
Related: aquarium salt vs Epsom salt.
How this guide is reviewed
This guide is written to be checked and signed off by a licensed small-animal veterinarian before it’s published. Until that review is complete, it’s marked pending review. Our aim is clear, owner-readable guidance that matches veterinary references, including telling you when the right move is to consult a licensed veterinarian, even though we’re a pharmacy.
Related fish & aquarium guides
Fish Dropsy · Fish Fin Rot · Fish Ich · Fish Popeye · Fish Columnaris · Fish Swim Bladder
Sources & References
Clinical statements on this page are drawn from veterinary references and guidelines. Full review by our veterinarian of record is pending.
- [1] Standard aquatic-medicine practice: isolating and treating sick fish in a separate hospital or quarantine tank protects the display system and allows accurate dosing (aquarium-medicine references; e.g. Smith, Fish Disease and Medicine).
- [2] Many aquarium medications are known to affect nitrifying (beneficial) bacteria and activated carbon removes dissolved medication from water; both are reasons to treat in a dedicated tank without carbon filtration.
For educational use. PrepperPetSupply is a licensed, veterinarian-staffed pharmacy; prescriptions are dispensed only after review by a licensed veterinarian and are not a substitute for in-person veterinary care. Not for human use.