Quarantine & hospital tanks

The quarantine tank — keeping disease out

Almost every outbreak arrives on something new: a fish, occasionally a plant, sometimes shared water or equipment. A quarantine tank breaks that chain. Hold new fish in it separately and watch. A hidden infection reveals itself where it can be dealt with cheaply and in isolation; a stressed, freshly-shipped fish gets time to recover and start feeding away from competition. It is the least glamorous and most effective practice in fishkeeping, and the authorities are unanimous: quarantine prevents far more loss than treatment ever recovers.

• Hold new fish apart and observe before adding them to the display
• Watch for the signs in the other guides; treat any problem in isolation
• Quarantine new plants too, and let shipped fish recover and feed
• Cheaper and far more effective than fighting an outbreak later

How long, and why

Two to four weeks is the common quarantine period, and the reasoning is worth understanding. The window needs to be long enough for a latent infection — one the fish is carrying without showing — to surface under the stress of shipping and a new home. For ich and velvet specifically, active disease often appears one to three weeks after a move, which is why around 30 days is widely recommended: it outlasts those incubation windows. Some keepers go further and treat new fish prophylactically during quarantine; others observe and treat only if something appears. Both approaches are defensible — observation avoids needless medication and the resistance it can breed, while prophylaxis trades that against extra certainty. The non-negotiable part is the waiting and watching.

Prophylactic treatment also carries a resistance cost that experienced keepers take seriously. Running an antibiotic against a fish that may not be bacterially infected selects for resistant strains; doing it routinely across multiple purchase batches concentrates that selection pressure. Reserve empiric medication for fish showing signs, and when a fish fails to respond or the diagnosis is genuinely unclear, a skin or gill scrape examined under a compound microscope — or submission to a fish-health laboratory — is far more informative than escalating to a second or third drug. Many diseases that look identical to the naked eye (flukes vs. velvet vs. bacterial gill disease, for instance) require microscopy to distinguish.

• Two to four weeks is typical; ~30 days covers ich and velvet incubation
• Long enough for a latent infection to surface under shipping stress
• Observe-and-treat vs prophylactic treatment — both are reasonable
• Routine empiric antibiotics breed resistance — treat only on clinical indication
• When response is poor or diagnosis is uncertain, microscopy or a lab beats shotgunning meds

The hospital tank — treating safely

When a fish falls ill, a hospital tank lets you treat it without dosing the entire display. That protects plants, shrimp, snails and — critically — the beneficial bacteria in the main filter, many of which medications harm. It also lets you give the patient pristine, stable conditions and dose accurately to a known volume. Hospital tanks are kept bare-bottomed and simply furnished (a hiding place, a gentle sponge filter) so they are easy to keep clean. One caveat from the triage guide: for a contagious disease already established in the display for days, moving only the sick fish treats the symptom, not the outbreak — there, the display itself is the patient. Match the tool to the problem.

Treatment contraindications matter as much as choosing the right drug. Salt at therapeutic levels is tolerated poorly by scaleless or lightly-scaled fish — loaches, many catfish, eels, and some tetras; what harms a parasite can also harm the patient. Copper-based treatments wipe out shrimp, snails, and most invertebrates, and linger in substrate and silicone long after water changes; the hospital tank must be clean and bare. Formalin drops dissolved oxygen significantly — at the concentration used against external parasites, an airstone is not optional. Many antibiotics are broad-spectrum enough to crash the biological filter in the hospital tank itself, so monitor ammonia during and after any antibiotic course. Temperature interactions also constrain choice: heat accelerates the ich life cycle and can help break the parasite phase faster, but the same elevated temperature worsens columnaris (Flavobacterium columnare), which progresses more virulently above 75 °F — raising temperature on a fish with saddle-back or fin-fraying lesions requires a confirmed diagnosis first.

Honest prognosis: some presentations are not recoverable, and the aquarist who recognises that early avoids prolonged suffering. A fish with advanced internal bacterial infection, late-stage systemic disease, or severe structural damage to gills or eyes rarely returns to full function regardless of treatment. Where recovery is genuinely unlikely, humane euthanasia — using clove oil at an appropriate concentration — is the responsible call. An aquatic veterinarian or a fish-health laboratory can help confirm a diagnosis when the presentation is ambiguous, treatment has failed, or you need to know whether the rest of the system is at risk.

Biosecurity — the cross-contamination you don't see

A quarantine tank only works if pathogens cannot walk around it on your equipment. Many fish diseases spread on shared gear: the sticky environmental cysts of ich cling to nets, siphon hoses, and hands, and a splash of water carried between tanks moves parasites and bacteria just as effectively as moving a fish. The discipline that closes that gap is simple but easy to neglect — keep dedicated nets, siphons, and buckets for the quarantine and hospital tanks, or disinfect and dry equipment between uses, and never let water from one system drip into another. It is the same biosecurity logic aquaculture facilities formalise, scaled down to a home fishroom: the goal is to make each tank an island.

• Pathogens ride on shared nets, siphons, hands and splashed water
• Keep dedicated equipment per tank, or disinfect and dry between uses
• Don't let water move between systems
• Treat each tank as an island — that's what makes quarantine real

Running them well

Neither tank needs to be elaborate: a modest tank, a heater, a gentle filter — ideally a sponge filter kept partly cycled on the main system so it is already colonised by bacteria the moment you need it — and a lid. Water quality should be immaculate in both: small, frequent water changes, because clean stable water is most of what makes quarantine observation reliable and treatment effective, and because a bare hospital tank has little biological buffer of its own. A low level of aquarium salt is sometimes used as gentle support, within the tolerance of the species. When a course of treatment ends, clean and dry the tank and all equipment before the next use so nothing carries over to the next occupant.

One practical trap: if you dose the hospital tank with an antibiotic and the sponge filter was the only cycled media you had, that filter may emerge from treatment with a crashed bacterial population — test ammonia before returning the fish to the main system. Running two sponge filters on the main display means one is always available without leaving the hospital tank unfiltered. Know when to call in help: an aquatic vet or a university fish-health lab is the right move when a disease is unresponsive after a correct first-line treatment, when you have cluster mortality without a clear cause, or when you need a confirmatory scrape or culture before committing to a treatment that would compromise shrimp, plants, or biological filtration in a display you cannot tear down.

• Keep a spare sponge filter cycled on the main tank so a tank is ready fast
• Run two cycled sponge filters on the display — one always available without leaving the hospital tank bare
• Bare-bottom, simply furnished, with a lid and a heater
• Immaculate water quality via small, frequent changes; monitor ammonia during and after antibiotics
• Clean and dry between uses
• Escalate to an aquatic vet or fish-health lab when treatment fails, mortality clusters, or diagnosis is genuinely uncertain