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

Why you actually need one

Most fishkeepers don't set up a hospital tank until something goes wrong. By then, ich has spread to every fish in the display, or a new arrival has brought in velvet and wiped out a tank they spent years building. A hospital tank is cheap insurance against both scenarios.

The two jobs overlap enough that one tank handles both:

• Quarantine: new fish go in here for 4-6 weeks before joining your display tank. You're watching for disease signs, parasites, and behavioural issues before they become your whole tank's problem.
• Treatment: when an established fish gets sick, you pull it out, treat it here, and leave your main tank undisturbed.

Treating in the display tank is almost always the wrong call. Medications like copper-based treatments and most antiparasitic drugs harm invertebrates and scaleless fish at therapeutic doses. Many antibiotics damage your biological filter, causing an ammonia spike on top of a disease outbreak. You end up treating fish that aren't sick, at diluted concentrations (because planted tanks and substrate absorb meds), and potentially crashing the tank in the process.

What you need - and what you don't

Tank size

For a single fish or a small group being quarantined, 40-75 litres (10-20 US gal) covers most situations. You don't need anything bigger unless you're quarantining a large species or a full group. Smaller water volume also means medication doses are easier to calculate accurately - a real advantage when you're working with treatments that have narrow therapeutic windows.

Bare bottom only. Gravel and sand trap uneaten food and detritus, harbour pathogens, and absorb medications. You want to be able to see waste, siphon it out completely, and know your meds are staying in the water column.

Filtration

A sponge filter is the right choice here. It's gentle on weak or recovering fish, won't trap small fish, is easy to disinfect between uses, and - most importantly - contains no chemical media. A small hang-on-back with the carbon removed works too. Avoid anything with built-in carbon cartridges that you might forget to swap out.

Do not use carbon when medicating. This is the single most common reason medication fails. Carbon adsorbs most drugs from the water within hours. If your filter has it, remove it before you add any treatment.

Heater

Match the temperature to the species you're treating. A small adjustable heater (50-100W depending on tank size and ambient room temperature) is fine. Stability is what matters - sick fish are already stressed and don't need temperature swings on top of everything else.

Lid

Non-optional. Stressed fish jump. A sick fish with no energy will still clear the waterline if it's startled. A simple glass or plastic lid works perfectly.

Hides

One simple hide - a short length of PVC pipe, an unglazed terracotta pot on its side, or a plastic cave - gives the fish somewhere to feel secure without creating waste traps or surfaces that are hard to disinfect. You don't need plants, decor, or substrate.

The cycling problem - and how to solve it

This is where most hospital tank setups fall apart. The tank sits empty for months, the filter has no ammonia source, the beneficial bacteria die off, and when you finally need it for a sick fish you have an uncycled tank. A stressed, sick fish then gets hit with an ammonia spike on top of whatever it's already dealing with. Ammonia at even 0.25 ppm causes gill damage and immune suppression - exactly what you don't want during treatment.

You have three realistic options:

Option 1: Spare sponge in the main tank (recommended)

Run a second sponge filter, or just a mature sponge, inside your main tank's filter or sump. It colonises with bacteria continuously. When you need the hospital tank, you pull the sponge out and drop it in - instant cycled media. This is the simplest and most reliable approach. The sponge just sits there costing you nothing until the day you need it.

Option 2: Keep a small permanent quarantine tank running

Some fishkeepers run their quarantine tank continuously with a couple of robust fish (endlers, a spare guppy, a small group of ember tetras). The tank stays cycled, and you can pull the resident fish out when you need to use it for treatment. It doubles as a holding tank for fish you're not sure about yet. The downside is ongoing maintenance on a second tank.

Option 3: Bottled bacteria plus frequent water changes

Bottled nitrifying bacteria products vary in quality, but the better ones (Seachem Stability, Dr. Tim's One and Only) can establish a partial cycle quickly enough to help. Combine with daily or twice-daily water changes and test for ammonia and nitrite every day. This is your fallback if you need the tank urgently and don't have a seasoned sponge available. Understand the nitrogen cycle before relying on this approach - you need to know what you're testing for and why.

How to use it: quarantine for new fish

New fish go into the hospital tank before they go anywhere near your display. No exceptions, even if the fish "look healthy." Ich, Velvet (Oodinium pillularis), and internal parasites are frequently invisible in early stages or while the fish is stressed from transport.

The quarantine period is 4-6 weeks. Four weeks catches most parasitic infections with visible symptoms. Six weeks gives you a reasonable buffer for slower-developing issues and lets you observe feeding behaviour, body condition, and whether the fish has any ongoing problems.

During quarantine:

• Feed small amounts and siphon waste daily.
• Test ammonia and nitrite every 2-3 days minimum - more often if the tank isn't fully cycled.
• Do water changes as needed to keep both at zero.
• Watch for white spots (ich), gold dust appearance (velvet), clamped fins, abnormal swimming, hollow belly, or laboured breathing.
• Don't add the new fish to the display until the full quarantine period is up and the fish are eating well and behaving normally.

How to use it: treating sick fish

When a display tank fish shows signs of illness, move it to the hospital tank promptly. The longer a sick fish stays in the main tank, the longer any pathogen has to spread.

Before you treat

Identify what you're actually treating before you buy medication. Treating bacterial infection with an antiparasitic, or vice versa, wastes time and stresses the fish further. Common presentations:

• White spots, scratching: Ichthyophthirius multifiliis (ich). Treat with heat (raised to 30°C / 86°F for species that tolerate it) plus formalin/malachite green, or copper-based treatments.
• Gold dust, rapid gill movement: Velvet (Oodinium). Requires copper treatment in the dark (velvet is photosynthetic).
• Fin erosion, red streaking: Bacterial - consider antibiotics appropriate to your region's available medications.
• Bloating, pinecone scales: Dropsy. Indicates organ failure, often internal bacterial infection. Prognosis is poor; treatment with Epsom salt and antibiotics is palliative in many cases.

Medicating correctly

Follow dosing instructions for the tank's actual volume - measure it, don't estimate. Remove carbon and any other chemical filtration before dosing. Complete the full treatment course even if the fish looks better partway through; stopping early is a reliable way to have the disease return.

After treatment, run fresh carbon for 48-72 hours to remove medication residue before returning the fish to the display. Do a 30-40% water change first.

What to do with the main tank during treatment

If the sick fish came from a display with tankmates, those tankmates may be exposed. For parasites like ich, the free-swimming stage is in the water regardless of which fish show symptoms. Running the display at 28-30°C (82-86°F) for 2 weeks (if all your species tolerate the heat) and doing large water changes to reduce pathogen load is often worth doing alongside treating the isolated fish.

Check that your water parameters are correct in the main tank too. Disease outbreaks frequently follow a parameter problem - temperature fluctuation, ammonia spike, pH crash - that stressed the fish first. Fix the root cause, not just the symptom.

Between uses: storage and hygiene

After use, disinfect the hospital tank thoroughly. A dilute bleach solution (1 part household bleach to 9 parts water, 10 minutes contact time) followed by a thorough rinse and dechlorination soak kills most pathogens. Let everything dry completely before storing. Replace your cycled sponge back in the main tank to keep it active.

Keep a dedicated set of equipment - net, siphon tube, bucket, thermometer - for the hospital tank only. Label them. Cross-contaminating tools between tanks is a fast way to spread the exact thing you were trying to contain.

What most guides miss

A few things that don't get enough coverage:

• Temperature matching before transfer: Float the sick fish's transport container in the hospital tank for 15-20 minutes before releasing it. A temperature shock on top of disease stress can kill a fish that might otherwise have survived.
• Light levels: Dim or no lighting reduces stress on sick fish and is essential if you're treating velvet. A dark environment is part of the treatment protocol, not just a comfort measure.
• Salt is not a universal cure: Aquarium salt has legitimate uses (electrolyte replacement, mild osmotic stress on some parasites) but it doesn't treat ich effectively at realistic doses, does nothing for bacterial infection, and harms scaleless species and many soft-water fish. Use it when it's indicated, not as a default.
• Observe before you medicate: Sometimes a fish that looked sick in the store is just stressed from transport and recovers in clean, stable water with good food. Give it 48 hours before reaching for medication unless symptoms are clearly progressive.

If you're building out a new tank and thinking about which fish to start with, choosing hardier species reduces how often you'll need the hospital tank - but you'll still need it eventually. Understanding how to cycle a tank properly applies directly to keeping the hospital tank functional when you need it fast.