The single biggest thing separating aquarists whose fish rarely die from those whose fish frequently do is a quarantine tank. £50 of kit prevents years of disease outbreaks, medication mistakes, and community-tank catastrophes.

This guide covers exactly how to set up a UK quarantine tank, when to use it, and why every aquarist should have one — even if they don’t buy new fish often.

Why quarantine matters

New fish from any UK aquatic shop arrive with some risk of:

• Ich (white spot) — often latent, appears within 2 weeks of stress
• Velvet — similar profile
• Internal parasites — camallanus worms, tapeworms, protozoa
• Bacterial infections — columnaris, fin rot, dropsy
• Fungal infections
• Neon Tetra Disease (in tetras specifically) — see our neon tetra guide
• Dwarf Gourami Iridovirus — see our dwarf gourami guide

Add fish directly to your main tank, and any of these spread to your entire community. Cure treatments are stressful for healthy fish AND kill invertebrates like shrimp.

A quarantine tank isolates the risk. Sick fish stay in quarantine; you treat there without endangering the community.

What a quarantine tank looks like

Simple. Bare. Functional.

• Size: 30–50L (smaller = harder to maintain water quality; bigger unnecessary)
• Bare bottom — no substrate. Easier to remove waste; less medication absorbs into sand/gravel.
• Sponge filter with air pump — cheap, safe, gentle
• Heater — matched to species need
• A few plastic decorations — hiding places, easy to disinfect
• No live plants — most medications kill plants; also complicates the setup
• Lid — mandatory. Fish jump when stressed.

Total cost: £50–£80 for basic quarantine setup.

The kit list

• Tank: cheap 30–50L glass tank. 40L quarantine tank — £30.
• Sponge filter + air pump — £20.
• Adjustable heater (50W) — £15.
• Digital thermometer — £5.
• Plastic hiding places — £5–£10 (aquarium plastic caves, PVC pipe pieces).
• Seachem Prime dechlorinator — you already have this.
• API Freshwater Master Test Kit — you already have this.

Cycling the quarantine tank

Two approaches:

Approach 1 — Instant cycle by borrowing from main tank

Fastest and best. When you know you’re adding fish soon:

1. 1 week before the fish purchase: put the sponge filter in your MAIN tank alongside the existing filter. Beneficial bacteria colonise the sponge over the week.
2. On collection day: move the seeded sponge to the quarantine tank. Fill quarantine with dechlorinated water at matching temperature.
3. Turn on air pump — sponge is now active biological filtration.
4. Add the new fish. Bacterial colony handles the ammonia they produce.

This gives you a functional cycled tank without waiting 4–6 weeks.

Approach 2 — Full fishless cycle

If you set up quarantine before you’re ready for new fish:

• Fill with dechlorinated water
• Add pure ammonia to 3 ppm
• Cycle exactly as for a normal tank — see our cycling guide
• Takes 4–6 weeks

Once cycled, you can leave the quarantine tank running empty with occasional ammonia dosing to keep the bacterial colony alive between uses.

When to use the quarantine tank

Use case 1 — New fish acclimation and observation

Standard protocol: 4–6 weeks in quarantine before joining main community.

• Week 1: acclimate to your water (see how to acclimate new fish)
• Weeks 2–3: observe for symptoms of ich, velvet, other diseases
• Week 4: treat prophylactically if any concerns (praziquantel for parasites, general antibacterial)
• Weeks 5–6: continued observation; move to main tank if clean

Why 4–6 weeks: most diseases show symptoms within 2 weeks; extended quarantine catches slow-developing conditions.

Use case 2 — Hospital tank for sick fish

If a fish in your main tank shows symptoms of disease:

1. Move the sick fish to quarantine
2. Treat there with appropriate medication (fin rot: Waterlife Myxazin; ich: see ich guide)
3. Community tank stays medication-free, protecting invertebrates and sensitive species
4. Return the fish once cured (2+ weeks past last symptom)

Use case 3 — Breeding fish

Move a breeding pair to quarantine for controlled spawning.

• Predators (other tank inhabitants) can’t eat eggs/fry
• Water parameters can be tuned specifically
• Fry can be observed and raised

See our cherry shrimp breeding guide and breeding corydoras guide for species-specific approaches.

Use case 4 — Aggression time-out

If a fish is bullying tank mates, move to quarantine for a few days. Others recover; often the aggressive fish loses its social position and integrates peacefully after reintroduction.

Daily maintenance during quarantine

• Daily test: ammonia, nitrite
• Every 2–3 days: 25% water change (quarantine tanks accumulate waste fast because bare-bottom)
• Daily observation: any signs of disease? Fish eating?
• Reduced feeding: stressed fish need less; feed sparingly

Cleaning the quarantine tank between uses

Between fish batches:

1. Empty tank
2. Scrub with clean water only (no soap ever)
3. Rinse thoroughly
4. Refill with dechlorinated water
5. Restart with the previous cycle sponge filter

After a disease outbreak:

1. Empty tank completely
2. Bleach solution (1:9 bleach:water) rinse for glass and decorations
3. Rinse thoroughly with dechlorinated water (dose 5× Seachem Prime to be safe)
4. Air-dry all removable parts for 24 hours
5. Restart with fresh sponge filter — the old one carried pathogens

Common mistakes

Only setting up quarantine when needed. By then it’s too late — you can’t quickly cycle a tank.

Not using it because “just this fish should be fine”. The one time you skip is when trouble arrives.

Using main-tank water in quarantine. Cross-contamination goes both ways. Use fresh dechlorinated water.

Overstocking quarantine. 3–4 fish max in a 40L quarantine tank. Any more and the water quality crashes.

Feeding the same amount as main tank. Stressed fish eat less; overfeeding fouls water rapidly.

The three-tank rule of thumb

Serious UK aquarists often run three tanks:

1. Main tank — the community display
2. Quarantine tank — for new fish
3. Hospital tank — for sick fish (sometimes the same as quarantine, used at different times)

For a hobbyist with 2+ years experience, this is entirely normal. It’s the mark of someone who has learned from lost fish that prevention is cheaper than cure.

Budget approach — the “quick-start” quarantine

For someone who doesn’t want a permanent quarantine tank:

• Store a bare-bottom aquarium in a cupboard, with all equipment
• Keep a sponge filter permanently seeded in the main tank (“seed sponge”)
• Set up quarantine only when needed
• Set-up time: 30 minutes

Cheaper than a permanent quarantine but requires you to always know when new fish are coming.

Bottom line

Every UK aquarist with a community tank should have quarantine capability.

Minimum kit: 40L bare tank + sponge filter with air pump + heater + dechlorinator + test kit. About £50 total.

Set up. Cycle (either seed a sponge in your main tank, or fishless-cycle empty). Use for 4–6 weeks per new fish batch. Save your main community from disease crashes.

The single best £50 you’ll spend in the hobby.

Frequently asked questions

Do I really need a quarantine tank?

Yes if you value your community. Quarantining new fish for 4–6 weeks catches diseases like ich and velvet before they reach your main tank, where treatment endangers shrimp and sensitive species.

How do I set up a quarantine tank?

Use a bare 30–50 litre tank with a sponge filter, heater and a few plastic hides. Seed the sponge filter in your main tank for a week beforehand so it is instantly cycled.

How long should fish stay in quarantine?

4–6 weeks. Most diseases show symptoms within two weeks, and the extended period catches slower-developing conditions before fish join the community.

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Related: how to acclimate new fish, ich treatment, fish disease overview, fin rot treatment, how to cycle a fish tank.

— Jaqline Smalling, FishForever