A quarantine tank is not glamorous. It sits in a spare room, a garage, or under a desk, and it spends most of its life empty. Yet it is the single most useful piece of kit a marine keeper can own. New fish carry parasites and bacteria. A scrape or a bullying incident can turn into a bacterial infection overnight. Without a separate tank, your only option is to treat the whole display, which is rarely practical in a reef. Setting one up is straightforward, and it does not need to be expensive.
Choosing the tank and where to put it
Size matters less than stability. A 40–60 litre tank (roughly 10–15 UK gallons) suits most small and medium marine fish. Tangs, large angels, and triggers need more room, but a bigger tank is harder to keep stable if you are using medications. Go for the largest tank you can comfortably manage and site it away from direct sunlight and household traffic.
Use a bare-bottom tank. No sand, no live rock. Sand traps waste and absorbs copper treatments. Live rock does the same, and it also dies back when you use medication, fouling the water. Instead, give fish a few lengths of PVC pipe or plastic plants for cover. A tight lid or mesh cover is essential: marine fish jump, especially when stressed.
Filtration, heating and water movement
Simple is best. A sponge filter driven by an air pump provides biological filtration and gentle flow. Add a small powerhead or wavemaker to break the surface and keep oxygen levels up. If you use a power filter, choose one with a sponge or ceramic media rather than carbon, because carbon can remove some medications.
To avoid an ammonia spike, keep a spare sponge in your display sump for a few weeks before you need it. When you set up the quarantine tank, that sponge is already colonised with bacteria. If you do not have a sump, use a bottled bacteria product and test daily.
Heating: an adjustable 100W heater with a guard is fine for most 40–60 litre tanks. Set it to 25°C (77°F) for most tropical marine fish, but check the needs of your species. Use a separate thermometer to verify. Lighting can be a simple LED unit on a timer. Bright light is not necessary and can stress fish; a subdued light helps you observe without spooking them.
The UK equipment checklist
- Tank, 40–60 litres or larger, with lid or mesh cover
- Sponge filter or small internal filter, plus air pump and air line
- Small powerhead or wavemaker
- Adjustable heater, 100W, with guard
- Thermometer (digital or glass)
- Ammonia alert badge (a useful backup)
- Test kits: ammonia, nitrite, nitrate, pH, salinity
- Refractometer (more accurate than a hydrometer)
- Copper test kit if you plan to use copper-based medication
- PVC elbows or plastic plants for hiding places
- Dedicated bucket, siphon, and net — never share with the display tank
- Medications: see below
Medications and treatments to keep on hand
You cannot diagnose every problem in advance, but a few basics cover most situations. Store them in a cool, dark place and check expiry dates.
- Copper-based treatment for marine ich and velvet. Copper is toxic to invertebrates, so it must never go in a reef tank. Use a chelated copper product and test the level daily with a copper test kit. Follow the manufacturer's dose exactly.
- Praziquantel for flukes and internal worms. It is relatively gentle and can be used in quarantine. Some fish may stop feeding for a day or two.
- Antibiotics for bacterial infections. In the UK, many antibiotics for fish are prescription-only. If you suspect a bacterial infection, contact a vet experienced in fish health rather than guessing.
- Formalin or freshwater dips for external parasites. These are harsh treatments. Do not use them unless you are confident in the diagnosis and the dose. When in doubt, seek advice.
Do not mix medications unless the label says it is safe. Do not use copper with praziquantel at the same time unless you have checked compatibility. And never treat the display tank with copper or antibiotics; the rock and sand will absorb them, and the invertebrates will die.
Running the quarantine period
- Set up the tank and fill it with fresh saltwater at the same salinity and temperature as your display. Run the filter and heater for 24 hours.
- Acclimatise the new fish as you would for the display, then release it into the quarantine tank. Keep the lights low for the first day.
- Observe daily. Feed a good quality frozen or pellet food. Remove uneaten food and test ammonia and nitrite every day for the first week, then every two or three days.
- If the fish shows symptoms — white spots, rapid breathing, clamped fins, flashing — treat according to the diagnosis. If you are unsure, ask an experienced keeper or a fish vet.
- If no symptoms appear, keep the fish in quarantine for at least four weeks, ideally six. This gives slow-developing parasites time to show themselves.
- If you had to treat for a disease, finish the treatment and then observe for a further two to four weeks after the last symptom disappears.
During quarantine, do not share nets, siphons, or buckets with your display tank. Wash your hands. Keep a towel dedicated to the quarantine area. These small habits prevent cross-contamination.
Making quarantine a habit
The best quarantine tank is the one that is always ready. Keep the sponge filter seeded in your sump. Keep a spare heater and a bottle of dechlorinator. When a new fish arrives, you can set up in an hour. It is a small effort that protects every fish in your display, and it turns a crisis into a controlled procedure. If you are ever unsure about a diagnosis or treatment, especially where prescription medicines are concerned, speak to a vet who works with fish. Your local aquarium club or a reputable marine shop can often point you in the right direction.
Photo: Vanessa Loring / Pexels
