Quarantine is the least glamorous subject in fishkeeping and the one with the best return on effort. Nobody sets up a quarantine tank because it is fun. You set one up after the first time a bag of new fish brings ich into a display tank you have run cleanly for two years, and you spend three weeks treating every fish you own because of six new tetras. Most keepers learn it that way. You can learn it the cheap way instead, which is what this article is for.

Why bother

Shop fish are stressed fish. They have usually been netted, bagged, flown, held at a wholesaler, shipped again, and dumped into a shop system that shares water across dozens of tanks, all within a few weeks. Stress suppresses their immune systems exactly when they are being exposed to every pathogen passing through the trade. A fish can look perfect in the shop and be carrying ich, velvet, gill flukes or an internal worm load that only shows itself a week or two later, once it has had time to develop or once the stress of one more move tips the fish over.

None of that is the shop’s fault, even a good shop. It is the nature of moving live animals around the world. The question is only where the problem surfaces: in a bare 40 litre tank holding six new fish, or in your fully stocked display where every resident is now exposed and where catching anything again means dismantling the scape.

Quarantine also protects the new fish themselves. They arrive thin and rattled, and a quiet tank with no established residents chasing them off the food gives them a few weeks to feed up and settle before they have to hold their own.

The minimum kit

A quarantine tank does not need to be pretty, and it should not be. Mine is deliberately plain, because plain is easy to observe and easy to sterilise. The full list:

  • A small tank, 40 to 60 litres for most community fish. A plastic storage tub of similar size works too.
  • Bare bottom, no substrate. Waste and uneaten food sit in plain view and get siphoned in seconds, and there is nowhere for parasite stages to hide.
  • A simple air-driven sponge filter. Cheap, gentle, and unaffected by most medications.
  • A heater and a thermometer.
  • Some cover so the fish do not feel exposed: a couple of pieces of PVC pipe, a plastic plant, a floating bit of hornwort. Things you can bleach or bin.
  • A tight lid. New fish jump far more than settled fish.
  • Its own net and siphon that never touch your other tanks.

The whole setup costs less than a decent shoal of fish. That framing matters: quarantine kit pays for itself the first time it stops one outbreak.

The clever trick that makes it painless is to keep a spare sponge filter running in your main tank all the time. When new fish are coming, you move that sponge into the quarantine tank and it arrives fully cycled, carrying an established bacterial colony. Instant safe quarantine, no ammonia spike. When quarantine ends, sterilise the sponge or leave the tank running fishless for a few weeks, then start again.

If you get caught without a cycled sponge, you can still run quarantine, you just have to work harder: test for ammonia frequently and do large daily water changes to keep it near zero. A bare tank makes big water changes trivially quick, which is another reason for the bare bottom. Whatever happens, do not skip quarantine because the tank is not cycled. An uncycled quarantine with daily water changes still beats no quarantine.

Two to four weeks of watching

I quarantine new arrivals for a minimum of two weeks, and I prefer four when I can hold my nerve. The reasoning is simple: the common parasites have life cycles measured in days to a couple of weeks, and stress-triggered problems usually surface within the first fortnight. A fish that has fed well and looked clean for four weeks is a good bet.

What you are actually watching for, at feeding time each day:

  • Ich (white spot). Distinct white grains, like salt, on fins and body. Often preceded by fish flicking or rubbing against objects.
  • Velvet. A finer, dusty, gold or grey film, easiest to see with a torch shone along the fish in a dark room. Fish flick, clamp fins and breathe fast. Velvet kills faster than ich and is missed more often.
  • Flukes. Nothing visible on the fish, which is the trap. Look for behaviour: scratching, sudden darting, heavy breathing, sitting under the filter outflow, clamped fins. Gill flukes are common on farmed livebearers and fancy goldfish.
  • Internal trouble. White stringy faeces, a hollow belly on a fish that eats, or a fish that eats and spits. These point at internal parasites or worms.
  • General condition. Fin edges, cloudy eyes, red streaks, ragged slime coat, and whether every single fish comes to food.

Keep the lights modest and the tank quiet, and resist the urge to medicate healthy-looking fish “just in case” with harsh general cures. Clean water, good food and observation are the default. Medicate when you see something, and then treat the specific problem properly, at full course length, in the quarantine tank where dosing is accurate because the volume is known and there is no substrate or carbon soaking up the medicine. Treating a bare 40 litre tank is easy and cheap. Treating a scaped 200 litre display with sensitive plants, shrimp and snails is neither.

Acclimatising new arrivals

Getting fish from bag to quarantine matters too. Float the bag for fifteen minutes or so to match temperature, then add small amounts of tank water to the bag over twenty to thirty minutes before netting the fish out and discarding the bag water. Never pour shop water into any of your tanks; it carries whatever the shop system carries.

Go gently if your water is markedly different from the shop’s. Fish moving between very different hardness or pH need a slower drip acclimatisation over an hour or more. This is one more argument for buying fish suited to your tap water in the first place, which I have written about in the hard water fish article. A fish that matches your water arrives with one less stress to absorb.

Feed lightly on day one, or not at all. A rattled fish will not eat, and the food just fouls the water.

The habit, honestly

I will not pretend I have never broken my own rule. Every experienced keeper has, and most of the disease stories they tell start with “I skipped quarantine just this once”. The maths is not subtle. A quarantine setup costs about as much as one bag of nice fish and takes ten minutes to deploy. One outbreak in a display tank costs weeks of treatment, usually some losses, and occasionally the whole stock. Boring habits are how tanks stay boring, and a boring tank is the goal.