Acclimating & Quarantining Marine Fish: Ich, Black Ich, Velvet & More

More fish are lost in the first few weeks after purchase than at any other point in a reef tank’s life — not because the fish were unhealthy, but because of how they were acclimated, or because a parasite nobody caught made it straight into the display. Getting acclimation and quarantine right, and knowing how to recognize and treat the handful of diseases that account for most losses, is the difference between a new fish settling in and a tank-wide outbreak.

Drip Acclimating New Fish

Float the sealed bag in your tank for about 15 minutes to match temperature, then pour the fish and its bag water into a clean bucket — never pour bag water into your tank or quarantine system. Run a length of airline tubing from your tank to the bucket, start a siphon, and control the flow to roughly 2–4 drops per second (a loose knot in the line works fine if you don’t have a valve). Let the volume in the bucket roughly double, which usually takes 30–60 minutes for most fish and up to an hour or two for more sensitive species. Net the fish out of the bucket into the tank and discard the water — the slow drip gives the fish time to adjust to differences in salinity, pH, and temperature without the shock of a sudden change.

Freshwater Dips for Fish

A freshwater dip for fish works on the same principle as a coral dip: the sudden change in osmotic pressure kills soft-bodied external parasites like flukes almost instantly. Use dechlorinated water matched to your tank’s temperature and pH, and keep most fish in for 3–5 minutes — hardier fish like clowns and damsels can often tolerate up to 8 minutes, while delicate species like wrasses and butterflyfish should come out closer to 2–3 minutes. Watch the fish the entire time and pull it immediately if it shows severe distress. A freshwater dip knocks adult flukes loose, but it doesn’t kill their eggs or eliminate an infestation on its own — it’s a supplement to real quarantine and medication, not a replacement for it.

Why a Quarantine Tank Is Non-Negotiable

A large share of fish carry parasites even when they look perfectly healthy at the store, and most of the medications that actually work on those parasites are not reef safe — copper alone will kill your corals and inverts. Quarantining every new fish for 4 weeks minimum before it goes into your display does two things: it gives you a safe place to treat a problem without touching your reef, and it gives a hidden infection time to show itself before it ever reaches your main tank. Skipping quarantine because a fish “looks fine” is how outbreaks like velvet and ich end up wiping out an entire established display.

Marine Ich (Cryptocaryon irritans)

Marine ich shows up as small white salt-like spots scattered across the body and fins, often with fish flashing (scratching against rock) and breathing heavily. The three effective treatments are copper (dosed and tested daily, since it drops out of solution quickly), hyposalinity (lowering salinity to roughly 1.008–1.009 specific gravity, dropped slowly and held for at least 4 weeks after the last spot disappears), and the tank transfer method, which moves fish to a fresh tank on a set schedule to physically separate them from the parasite’s free-swimming stage. All three require a dedicated quarantine tank — none of them are reef safe.

Black Ich (Turbellarian Flatworms)

Despite the name, black ich isn’t related to marine ich — it’s caused by parasitic flatworms that burrow into the skin and trigger dark, peppery black spots, most commonly seen on tangs. Praziquantel (found in products like Prazipro or API General Cure) is the most reliable treatment, dosed in quarantine for about a week. A 45–60 minute formalin bath is another option for a faster knockdown, followed by moving the fish into a clean quarantine tank. Hyposalinity held for at least 10 days also works, and a freshwater dip can offer short-term relief but won’t fully clear an infestation on its own.

Marine Velvet (Amyloodinium ocellatum)

Velvet is the most dangerous disease on this list — it’s fast-moving and can wipe out an entire tank of fish within days if it isn’t caught immediately. Look for a fine gold or rust-colored dusting on the skin, rapid gill movement, listlessness, and fish rubbing against rock or sand. Copper is the standard treatment, held at a therapeutic concentration of roughly 0.15–0.20 mg/L and tested daily, since the margin between an effective dose and a toxic one is narrow. Because of how fast velvet spreads and how lethal it is, any fish showing these symptoms needs to move to quarantine and start treatment immediately — don’t wait to see if it clears on its own.

Intestinal Parasites

Internal parasites are harder to spot than external ones — stringy white feces, a fish that’s eating well but staying thin, or a sunken belly are the usual signs. Treatment is done through medicated food rather than dosing the water: praziquantel handles intestinal worms, and metronidazole covers internal flagellates, and the two are often combined for broader coverage. Soak the fish’s regular food in the medication for about 30 minutes before feeding, and continue daily for 2–3 weeks or until symptoms clear.

Lymphocystis (“Cauliflower Disease”)

Lymphocystis is a viral infection that produces small white or beige growths on the fins and skin that clump together into a lumpy, cauliflower-like mass over time. It’s chronic rather than acutely dangerous — it rarely kills a fish outright, but it can weaken it and open the door to secondary bacterial infections. There’s no direct cure since it’s viral, and most cases regress on their own over weeks to months with stable water quality, good nutrition, and low stress. The best defense is prevention — a solid quarantine routine keeps it from ever reaching your display in the first place.

Treatment at a Glance

• Marine ich: copper, hyposalinity, or tank transfer method — quarantine required
• Black ich: praziquantel (about 1 week), or a 45–60 min formalin bath, or hyposalinity for 10+ days
• Marine velvet: copper at 0.15–0.20 mg/L, tested daily — treat immediately, it moves fast
• Intestinal parasites: praziquantel + metronidazole soaked into food, daily for 2–3 weeks
• Lymphocystis: no direct cure — supportive care and time, usually resolves on its own

The Bottom Line

Drip acclimate every new fish, quarantine every new fish for at least 4 weeks, and learn to recognize these diseases early — the difference between a five-minute freshwater dip and a full copper treatment is usually just how fast you caught it. None of these medications belong in your display tank, which is exactly why a dedicated quarantine setup is worth the extra ten gallons of space. Questions about setting up a quarantine system or what you’re seeing on a fish? Reach out, or browse our current stock while you’re here.

A Bare Quarantine Tank Is a Feature, Not a Downside

Resist the urge to add rock or sand to a quarantine setup — a bare-bottom tank with just a heater, sponge filter, and PVC hides makes it far easier to spot disease early, siphon out uneaten food and waste daily, and run medications like copper safely without them being absorbed or neutralized by live rock. It looks unglamorous, but that’s exactly what makes it effective as a diagnostic and treatment space.