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🩺 Fish health how-to

How to quarantine new fish

Quarantine is the cheapest insurance in the hobby. A few weeks in a separate tank stops a single sick fish from wiping out an established aquarium. Here is how to do it simply and safely.

Why quarantine matters

Every new fish is an unknown. Even healthy-looking arrivals from a good shop can carry parasites or bacteria that only flare up once the fish is stressed by transport and a new environment. Add that fish straight into your display tank and you expose every fish you already own. Quarantine keeps the newcomer isolated until you are confident it is healthy, so one bad purchase does not become a whole-tank disaster. See our overview of whether you need to quarantine new fish.

It also gives the new fish something valuable: a calm, low-competition space to recover, feed and settle before facing established tankmates.

Setting up a quarantine tank

A quarantine tank is deliberately simple — you want something easy to observe and easy to clean, not a display piece.

  • A modest bare-bottom tank (no substrate) so waste and any medication are easy to see and manage.
  • A gentle, cycled filter — a sponge filter is ideal. Compare options on the filters hub.
  • A heater if you keep tropical fish, matched to the same temperature as your main tank — see the heaters hub.
  • A few hiding spots (plant pots, PVC pipe, silk plants) to reduce stress.
  • A test kit and dechlorinator for water changes.
Tip: Keep a spare sponge filter running permanently in your main tank. When you need to quarantine, move that sponge to the quarantine tank and it brings a ready-made bacterial colony with it — instant filtration, no waiting for a fresh cycle. Learn the cycle itself in our cycling guide.

The quarantine routine

Acclimatise the new fish into the quarantine tank exactly as you would any tank, using the slow method in our acclimation guide. Then settle into a two-to-four-week observation period.

  • Keep the lights dim and the tank quiet for the first days to reduce stress.
  • Feed lightly and remove uneaten food promptly.
  • Test the water regularly and do small water changes to keep ammonia and nitrite at zero.
  • Observe the fish closely every day and keep simple notes.

What to watch for

Daily observation is the heart of quarantine. Look for changes in behaviour and appearance, because early signs are easiest to treat. Common warning signs include clamped fins, rapid or laboured breathing, white spots, cotton-like growths, faded colour, loss of appetite, and unusual hiding or scratching against objects.

Our answer pages on telling if a fish is sick, ich (white spot) and fin rot can help you recognise and understand these. If disease appears, treat it in the quarantine tank following reliable, species-appropriate guidance, and extend quarantine until the fish is fully recovered.

Warning: This is general guidance, not veterinary advice. Diagnosing and medicating fish disease can be tricky, and the wrong treatment can do harm. When in doubt, research the specific condition thoroughly or consult an aquatic vet, and never dose medication into your main display tank "just in case."

Moving fish to the main tank

Once the fish has spent the full quarantine period eating well, behaving normally and showing no signs of illness, it is ready to join the display tank. Acclimatise it again to the main tank's water, add it during a calm part of the day, and keep an eye on how existing residents react. A short, disciplined quarantine now saves a great deal of heartache later — for more on stocking safely, see how many fish you can keep and the maintenance hub.

Frequently asked questions

How long should I quarantine new fish?

Two to four weeks is the usual guideline. Two weeks is a sensible minimum for observation; four weeks is safer because it covers the life cycle of common parasites and gives slower-developing problems time to appear. During that period you watch for signs of disease and let the fish recover from the stress of transport before they meet your main tank. If any illness shows up, the clock effectively resets until the fish is fully healthy again.

Does a quarantine tank need to be cycled?

Ideally yes. A quarantine tank should have stable biological filtration so ammonia and nitrite stay at zero while the fish is inside. The easiest way is to keep a sponge filter running permanently in your main tank, then move it to the quarantine tank when needed — it arrives already colonised with beneficial bacteria. If the tank is not cycled, you must test daily and do small water changes to keep the water safe.

Can I quarantine new fish in my main tank?

It defeats the purpose. The whole point of quarantine is to keep new arrivals separate so that any disease they carry cannot reach your established fish. Adding them straight to the display tank exposes your existing stock to whatever the newcomers brought with them. If you genuinely cannot run a separate tank, buy only from a trusted source, acclimatise carefully, and accept that you are taking a real risk.

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