Hospital tank: setup and use

A hospital/quarantine tank is an aquarist's insurance policy. Without one, treating a sick fish turns into a disaster: antibiotics destroy the biofilter in the main tank, copper kills the shrimp, methylene blue stains silicone. A dedicated tank is the only sane solution.

When you need it

New-fish quarantine

2–4 weeks of observation for each new fish before adding it to the community tank. Detailed protocol in our 'Quarantine new fish' guide. This tank often doesn't need medications at all.

Isolating a sick fish

When one fish gets sick in the main tank, move it here for treatment. The alternative (treating everyone in the main tank) costs more in meds, endangers the rest of the inhabitants, and destroys the bacterial colony.

Design: minimalism

Size

20–40 L for most small species. Cichlids, angelfish — 60 L+. The rule isn't 'what looks best' but 'what's easiest to treat'. A large volume makes medication dosing imprecise.

Bare bottom

Substrate absorbs medication, making the actual dose unpredictable. Gravel and aquasoil — definitely not. The floor stays bare glass: easy to clean and to count uneaten food.

Sponge filter

A key item. Internal or canister filters are out — they suck up weak fish. The sponge needs to be pre-cycled in the main tank (keep one there permanently as a 2+ week-old backup). That way the biofilter works from minute one.

Heater

Standard submersible 25–50 W. For ich treatment you need to be able to raise temperature to 30 °C — pick a heater with that headroom.

PVC hides

Plain 32–50 mm PVC pipes or fittings. They don't absorb medication, have no sharp edges, and disinfect easily. Decorative driftwood and ceramic ornaments — no: they absorb.

Lighting

Minimal: one weak lamp turned on only for inspection. Constant light stresses a sick fish. Many medications (metronidazole, methylene blue) break down under light.

What does NOT belong here

• Plants — most meds kill them, and they absorb a fraction of the dose. • Activated carbon — absorbs most medications, making treatment pointless. • Invertebrates (shrimp, snails) — copper and formalin are lethal to them. • Coated decorative sand — leaches unpredictable substances.

Keeping it 'ready'

Best practice — store the hospital tank dry, with all equipment on a shelf. Keep the filter sponge permanently in the main tank. When needed, the tank can be set up in an hour.

Alternative — keep it running with 2–3 hardy fish as a 'standby', but in an emergency you have to relocate them and disinfect first. Dry storage is simpler.

Medication compatibility

• Salt (NaCl) — harmless for most fish, lethal for corydoras, kuhli loaches, and some tetras. • Methylene blue — stains silicone, safe for fish, kills the biofilter. • Malachite green — toxic to scaleless fish (plecos, eels). • Formalin — toxic to all invertebrates; only in a quarantine without plants or shrimp. • Antibiotics (erythromycin, tetracycline) — wipe out the biofilter; expect heavy water changes.

Returning fish to the main tank

After the course of treatment: 1. 50 % water change in the hospital tank — no medication. 2. Wait 48 hours — watch for relapse. 3. Drip-acclimate to main tank water for 1 hour. 4. Transfer the fish without the hospital tank water (it may contain residual meds).

Decontamination after use

After hospital use, especially after a serious illness: • Drain fully. • Rinse the glass with hot water and 3 % hydrogen peroxide or 1:10 bleach solution. • Rinse 2–3 times with clean water. • Dry completely for 24 hours. • Don't disinfect the filter sponge — better discard it and seed a new one in the main tank.

A hospital tank isn't a 'temporary aquarium' — it's a standalone tool. Serious fishkeepers keep it ready like a fire extinguisher: cheap, on hand, and one day it saves a life.