You have done the research, the fish is in the bag on the passenger seat, and you already know which cave it is going in. Between the shop and your display tank there is one step plenty of reefers skip: quarantine. Not out of carelessness, usually, but because it feels like hassle. It is not. A modest plastic tank in a spare room is the difference between losing one fish and losing a system you have spent two years building.
Why a quarantine tank earns its keep
Most marine disease arrives quietly. White spot, velvet, flukes and internal worms are all carried by fish that look perfectly normal on the shop side of the glass. Shipping and handling suppress symptoms, so a fish can appear clean in the bag and break out in spots five days after it goes into your reef — by which point every other fish has been exposed.
Once something is in the display, your options shrink fast. Copper will wipe out your corals, inverts and live rock, so you cannot dose the reef. That leaves catching every fish, which means dismantling your aquascape, or running the tank fishless for weeks. Quarantine also gives a new arrival space to settle and start feeding without competing with established tankmates.
Setting up a quarantine tank
A simple 60-litre tank suits most small and medium fish; go larger for tangs, large angels or groups. Keep it deliberately plain — bare bottom, no live rock, no substrate. Rock and sand absorb copper-based medications and can harbour parasites between batches.
- Tank and lid. Marine fish jump, and a tight-fitting lid also slows evaporation.
- Heater and thermometer. Stability matters more than any specific temperature.
- Filtration. An air-driven sponge filter or a small internal filter is enough. Seed the sponge or ceramic media in your sump for a few weeks before you need it.
- Flow. A small powerhead or air stone for surface movement.
- Cover. Two or three lengths of PVC pipe, or a plastic plant pot on its side. Fish that feel exposed hide and stress.
- Test kits. Ammonia, nitrite, nitrate, pH and salinity as a minimum, plus a copper test kit if you plan to treat.
- Dedicated kit. A net, bucket, siphon and turkey baster that never touch the display tank.
Your biggest risk is ammonia. The tank is small, the filter is new, and one fish produces a surprising amount of waste. Test daily for the first fortnight, keep a bottle of ammonia detoxifier to hand, and be ready to do a water change at short notice.
Acclimating and settling the fish in
Float the bag for fifteen to twenty minutes to equalise temperature, then add tank water in small amounts over half an hour, or drip-acclimate if you prefer. Match the quarantine tank to the bag water first — many shop systems run at a lower salinity than a reef tank — then raise it slowly over several days. A jump of more than a point or two in specific gravity is hard on a stressed fish.
Keep the lights off for the first day, then offer a small amount of food. Frozen mysis or a quality pellet is fine. If the fish does not eat within two or three days, try a different food and check your water parameters before assuming illness.
The observation period: how long, and why
Four weeks is the sensible minimum. Six to eight is better, and many experienced keepers treat everything prophylactically rather than watching and waiting. The reason for the extra time is the parasite life cycle: a fish can look spotless while dormant stages sit in the water or on the glass, ready to reinfect days or weeks later.
Two rules keep this honest. First, the clock restarts if you see any symptom of disease. Second, it restarts if you add another fish to the same tank. Two fish bought a fortnight apart are not on the same schedule, and sharing a quarantine tank compromises both.
Observation or prophylaxis?
Observation means treating only when something appears. It avoids medicating healthy fish, but it relies on you spotting problems early, and some diseases move faster than that.
Prophylaxis means running copper at a therapeutic level for a set period, often alongside a praziquantel treatment for flukes and worms. It catches what you cannot see. The trade-off is that copper is hard on certain species, and the level has to be maintained — dropping below the therapeutic range for even a day can let parasites survive. Follow the manufacturer's instructions exactly and test daily.
Treating what you find
Match the treatment to the diagnosis rather than guessing.
- White spot and velvet: copper-based medication at the correct therapeutic level, or hyposalinity in a fish-only tank. Never in a reef.
- Flukes and worms: praziquantel, sometimes after a short freshwater dip matched for temperature and pH.
- Bacterial infections: do not throw antibiotics at a guess. Improve water quality, keep the fish feeding, and get proper advice.
Formalin-based products work but are toxic to you as well as to parasites — gloves, ventilation and careful measuring. Some medications that once sat on every shop shelf are now restricted in the UK, so check what you can legally buy before you need it.
Day-to-day care and cross-contamination
Disease rarely walks into a tank on its own. It arrives on wet hands, shared nets, drips from a siphon, even aerosols near the tank. Wash your hands between tanks and keep equipment entirely separate.
Change water with pre-mixed saltwater matched for temperature and salinity, and re-dose medication afterwards to replace what the change removed. Siphon uneaten food and waste, feed little and often, and resist the urge to add extra fish “to keep it company”.
Moving day and a simple routine to follow
Net the fish out rather than pouring the bag or tank water into the display. Once it is across, either strip and sterilise the quarantine tank or leave it running empty, ready for next time.
If disease does reach the display, the only reliable fix is time: leave the tank without fish for around six to ten weeks, longer being safer, so the parasite life cycle burns out.
- Set up and seed the quarantine filter before you buy a fish.
- Acclimate slowly and match salinity to the bag water first.
- Observe for a minimum of four weeks, restarting the clock after any symptom or new addition.
- Treat only with a matched medication, at the correct dose, testing daily.
- Use separate nets, buckets and hands for every tank.
- Transfer by net, never by pouring tank water into the reef.
It is not glamorous work. But a quarantine tank is the cheapest insurance in the hobby, and the fish you save from a wipeout will never know how close it came.
Photo: geraldrose / Pixabay
