Quarantining Marine Fish: Copper, Hyposalinity and Transfer
Marine ich and velvet are far deadlier than freshwater parasites. Learn observation quarantine, copper in a fish-only tank, hyposalinity and the tank-transfer method, and why corals can never share a copper QT.
Quarantining new marine fish is not optional in a reef-keeping mindset, because the parasites that ride in on imported fish are far more lethal than their freshwater counterparts. A separate quarantine tank lets you observe, diagnose and treat new arrivals with medications that would devastate a reef display. This guide focuses on the marine-specific parasites and protocols; it complements, rather than replaces, a general quarantine-tank setup.
Why marine fish need it
Two protozoan parasites dominate marine fish losses. Cryptocaryon irritans, the marine equivalent of white spot, has a life cycle that allows rapid intensification of infection. Amyloodinium ocellatum, marine velvet, gives fish a brownish-gold dusting and frequently causes high, fast mortality. Both spend part of their life cycle encysted, and while encysted they are refractory to chemical treatment, which is why every effective protocol relies on time as much as medication.
Observation quarantine
A minimum 30-day quarantine is the baseline, with longer periods often warranted. During this window, examine fish early and watch for spots, flashing, rapid breathing or the gold dusting of velvet. Quarantined animals should have their own dedicated nets, buckets and siphons so equipment never carries parasites into the display. Prophylactic treatment is often prudent for recently imported marine fish; praziquantel for flukes (monogeneans) is a common example.
Proven treatments
- Copper: the best-established therapy for Cryptocaryon and Amyloodinium, run as a measured, sustained exposure in a bare fish-only tank.
- Hyposalinity: lowering salinity to roughly 16-18 g/L helps control Cryptocaryon, held constant for at least three weeks; it does not reliably kill Amyloodinium.
- Tank transfer method (TTM): moving fish to fresh, clean tanks on a schedule so the free-swimming parasite stage is left behind before new cysts can release.
- Adjuncts such as chloroquine and formalin are used by some, though efficacy varies and these chemicals are themselves toxic to fish if mishandled.
Treat for long enough
Because the encysted stage shrugs off chemicals, treatment must run long enough to outlast every cyst that releases. Sustained exposure of at least three weeks is generally recommended for Cryptocaryon, and Amyloodinium typically demands repeated copper treatments to break its rapid reinfection cycle.
Why corals can't be in a copper QT
Copper, formalin and most fish medications kill invertebrates outright, so any inverts, plants and live rock must be removed before water is treated. This is the core reason marine fish are quarantined separately from corals: the treatments that save the fish would destroy the reef. Run a fish-only quarantine, complete the full course, and only then introduce healthy fish to the display.