Quarantine new fish

Why quarantine actually matters

Your established tank is a balanced ecosystem. The fish in it have adapted to your water, your bacterial colonies are stable, and whatever baseline pathogens exist are in equilibrium. Introducing a new fish without quarantine is how you break that equilibrium in the worst possible way.

Pet store fish carry pathogens more often than most hobbyists assume. Ich, Columnaris, velvet (Oodinium), internal worms, and bacterial infections regularly arrive on fish that look perfectly healthy in the shop. Stress from bagging, transport, and new water suppresses immune response, so a fish that appeared fine on the shelf can show symptoms within days of arriving home.

The harder problem is that even visibly healthy fish can be subclinical carriers - harbouring parasites or bacteria at levels too low to cause obvious symptoms, but high enough to infect your display tank's fish once introduced. There's no way to screen for this by eye. Quarantine time is the only reliable filter.

Setting up a quarantine tank

A quarantine tank doesn't need to be expensive or elaborate. Its job is to hold fish safely while you observe them, and to allow treatment without disrupting your main tank's cycle or exposing your established fish.

Tank size and equipment

A 10-20 gallon tank handles the majority of species most hobbyists keep. If you're quarantining large cichlids or fish over 6 inches, scale up accordingly. A bare bottom is standard - it makes cleaning easier, lets you spot waste and any shed parasites, and leaves nowhere for pathogens to hide in substrate. Add one or two pieces of PVC pipe or a plain plastic cave for hiding. Stress kills fish; cover matters.

You need a heater matched to the species you're quarantining. Tropical fish generally want 24-27°C (75-80°F), though species-specific requirements vary - always check the target range for what you're bringing home. A thermometer is non-negotiable.

Filtration - this is where most people fail

Running a quarantine tank with an uncycled filter is one of the most common mistakes in the hobby. You add a stressed, potentially sick fish, ammonia builds in a tank with no established bacteria, and within 48 hours you've got water quality problems on top of potential disease. Now you can't tell whether the fish is sick, ammonia-stressed, or both. Every treatment decision gets muddier.

The fix is simple: keep a spare sponge filter running permanently in your main tank. When you need to quarantine, move it straight over. It brings established bacteria with it and the quarantine tank cycles instantly. Alternatively, squeeze filter media or an old sponge from your main filter into the quarantine tank to seed it. See our guide on how to cycle a fish tank if you need to build from scratch - but a running sponge in the sump or main tank is easier long-term.

Water parameters

Match your quarantine tank's parameters to your main tank as closely as possible, particularly temperature and pH. Wild swings between the two tanks stress fish when you eventually move them. Aim for ammonia and nitrite at 0 ppm, nitrate under 20 ppm, and pH stable within the species' acceptable range. For a full breakdown of target values, see the guide on aquarium water parameters.

During quarantine, water changes are your primary tool for maintaining quality. With a lightly stocked quarantine tank, a 25-30% change every two to three days is a reasonable baseline - more if you're treating with medication, since many drugs break down quickly and need redosing after changes. Track your test results; App-aquatic's parameter log makes it easy to spot a trend before it becomes a crisis.

What to watch during quarantine

Observation is the whole point. Check your fish daily - ideally at the same time, before feeding, when the lights first come on. Baseline behaviour is your reference point; you're looking for anything that changes.

Physical symptoms

• White spots (salt-grain sized): Classic ich presentation. Check fins and body under good light.
• Gold or rust-coloured dust: Velvet (Oodinium). Often missed because it's subtle - shine a torch at a low angle across the fish.
• Clamped fins: Fins held tight against the body instead of extended. Non-specific but a reliable early stress/illness flag.
• Rapid gill movement: Can indicate gill flukes, low oxygen, or ammonia irritation - check water quality first.
• Sunken belly or pinecone scales (pineconing): Sunken belly often signals internal parasites or wasting disease; pineconing (scales raised outward) is a sign of dropsy, which has a poor prognosis.
• Lesions, ulcers, or frayed fins: Bacterial infection, often Columnaris or Aeromonas.

Behavioural symptoms

• Flashing: Rubbing or scraping against surfaces. Points to external parasites or gill irritation.
• Surface gasping: Rule out low oxygen or high ammonia before assuming disease.
• Hiding constantly after the first 48 hours: Some hiding is normal during initial acclimation. Persistent hiding beyond that warrants closer inspection.
• Refusing food after 48-72 hours: A newly arrived fish skipping meals for the first day or two is normal. Continued refusal past that is a red flag.

Treatment during quarantine

Quarantine gives you the ability to treat without medicating your entire display tank, killing beneficial bacteria, or exposing your other fish to drugs they don't need. Use it. But treat only when you've identified what you're dealing with.

When you do have a diagnosis, use a targeted treatment at the correct dose for the full recommended course. For ich, the most reliable approach is raising temperature to 30°C (86°F) for two weeks combined with copper or formalin treatment - temperature alone is rarely sufficient. For bacterial infections, antibiotics appropriate to the bacteria involved (gram-negative vs gram-positive matters). For internal parasites, metronidazole or praziquantel depending on the type.

After any medication course, run activated carbon for 24-48 hours and do a 30-40% water change before adding the next treatment or before considering the fish clear.

Prophylactic treatment - the ongoing debate

Some experienced fishkeepers treat every new fish prophylactically: a standard course of praziquantel for flukes and tapeworms, and metronidazole for internal parasites, regardless of visible symptoms. The reasoning is that internal parasites are almost impossible to detect visually, and treatment is low-risk if the fish is otherwise healthy.

Others argue that medicating healthy fish adds unnecessary stress and that careful observation is sufficient. Both positions have merit. If you're acquiring fish from sources with poor biosecurity - large wholesalers, chain pet stores - the prophylactic approach has a reasonable evidence base behind it. If you're buying from a trusted local breeder who knows the health history of their fish, watchful waiting is more defensible.

This is one of the genuinely contested areas of the hobby, and anyone telling you there's one right answer is oversimplifying.

How long is long enough?

Four weeks is the minimum. Two weeks - still common advice - catches ich and some bacterial infections but misses many internal parasites, which can take three to four weeks to present symptoms as the fish settles and the parasite load climbs. Some hobbyists run six-week quarantines as standard, especially for wild-caught fish or species known to be parasite-prone like discus (Symphysodon spp.) or wild bettas.

The clock restarts if you treat. A fish that shows ich on day 18, gets treated, and clears symptoms on day 25 should not be moved to the main tank on day 26. Wait another four weeks of symptom-free observation from the end of treatment.

Passing quarantine and moving fish

Before moving fish to your main tank, confirm all of the following:

• Eating consistently for at least two weeks
• No physical symptoms at any point during the final four weeks
• Active and displaying normal behaviour for the species
• Water parameters in the quarantine tank close to the main tank

When moving, acclimate carefully - the temperature and chemistry shift between tanks can still cause stress even when both are within normal ranges. A slow float or drip acclimation gives the fish time to adjust. Check your water change schedule and test your main tank parameters before introducing anything new, particularly if the quarantine tank ran any treatments that could have altered its chemistry.

After quarantine: disinfecting the tank

Once quarantine is complete, drain the tank, remove all equipment, and disinfect with a bleach solution (1 part bleach to 9 parts water) or potassium permanganate. Rinse everything thoroughly and let it dry completely before reuse. If you treated for ich or other parasites, the cyst stage can survive for weeks in moist conditions - don't shortcut the disinfection.

If you want the quarantine sponge filter to go back into the main tank, rinse it in dechlorinated water (not bleach) and return it. Bleach kills the bacteria; dechlorinated water preserves them.

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