Velvet disease: the gold dust you need a torch to see

What you are looking at

A fish that scratches itself on the gravel and the decor, holds its fins clamped, and breathes fast and shallow at the surface — and, if you look closely, a very fine dusting over its skin, gold, rust or grey, like someone blew powdered cinnamon over it. Most of the time you cannot see the dust at all in normal light. Turn the room lights off and shine a torch across the fish from the side, and the film glitters. That torch test is the whole diagnosis, and it is worth doing on any fish that is flashing and breathing hard with nothing else visible.

Velvet is a parasite — Piscinoodinium in freshwater, Amyloodinium in marine tanks, both dinoflagellates, distant relatives of the organisms that cause red tides. It has the same life cycle as ich: a feeding stage on the fish, a cyst stage on the substrate that divides into hundreds, and a free-swimming stage that must find a fish within a day or two. What makes it worse than ich is where it feeds. The gills go first, days before the skin shows anything, so by the time the dust is visible the fish has been struggling to breathe for a week. Untreated, it kills more reliably than ich and faster, and it takes the whole tank with it.

Ich is the thing to rule out, and the difference is the grain. Ich is distinct white spots the size of a grain of sugar, each one separate. Velvet is a film, far finer, and coloured — gold, yellow or rusty brown rather than white. A fish can have both.

Why the treatment is different from ich

Two things. The first is that the parasite is photosynthetic: the feeding stage carries chloroplasts and gets part of its energy from light. Blacking out the tank — lights off, a towel or cardboard over every side, for the whole course of treatment — starves it and is the single most effective non-chemical step, and it is one nobody thinks to take for ich.

The second is that the standard ich medications are only partly effective. Malachite green and formalin slow velvet but rarely clear it. Copper does. That matters because copper is unforgiving: the dose that kills the parasite is close to the dose that harms scaleless fish, and it kills shrimp, snails and most invertebrates outright at any dose that works. Every step below is built around that.

The treatment, step by step

1. Fix the water first. Velvet, like ich, blooms in a tank where something has stressed the fish. Test today. Any ammonia or nitrite above zero means a 30–50 % water change now and daily tests until both read zero; nitrate over 40 ppm gets the same. A copper treatment on top of an ammonia problem is two poisons at once.

2. Move the invertebrates and the plants you care about. Shrimp and snails go to a bucket or a spare tank with a heater and an air stone for the duration; they do not carry velvet in any meaningful way and copper will kill them. Live plants tolerate a short copper course poorly and a long one badly. If the tank is a planted shrimp tank, treat the fish in a bare hospital tank instead and leave the display tank fishless for a fortnight, which starves the parasite out.

3. Black the tank out. Lights off and the tank covered on all sides so no room light reaches it, for the whole course. Feed by lifting one corner of the cover. The fish do not mind; the parasite does.

4. Raise the temperature — slowly. To 28–29 °C at no more than 1 °C every twelve hours, if every fish in the tank tolerates it. Warmth shortens the life cycle, pushing the parasite into the vulnerable free-swimming stage sooner. Warm water holds less oxygen and velvet is already on the gills, so add an air stone or turn the filter outlet up to churn the surface.

5. Copper, dosed and measured. A copper-based aquarium medication (a chelated copper product is the gentlest), dosed for the tank’s actual water volume, with activated carbon removed from the filter first. Copper is one of the few medications you can and should test for: a copper test kit, kept at the level the label names, is the difference between a cure and a poisoning. Keep the level up for at least ten days after the last dust is gone, re-dosing for the volume of every water change. Scaleless fish — loaches, some catfish — get half the dose and a close watch, or a salt-and-heat treatment in a hospital tank instead.

6. Aquarium salt, for fish that tolerate it. One tablespoon per 20 litres supports the gills and the slime coat and stresses the free-swimming stage. Skip it for scaleless species and in a tank with live plants.

In the app — a checkup with a torch-lit photo and your readings gives the diagnosis and a treatment plan sized to your tank’s volume; Add plan to my schedule books the daily copper checks, the water changes and the day the lights can go back on, and the follow-up asks whether the flashing has stopped.

Run a checkup on the flashing →

Afterwards

When the course is done, a large water change and fresh activated carbon in the filter pull the copper out; run the carbon for a week and change it once. Test copper to zero before the shrimp and snails come home. Bring the lights back over a few days. And ask how it got in: velvet arrives on a new fish far more often than any other way, and four weeks in a quarantine tank — where the torch test is a two-minute evening habit — is how it never arrives again.

Common questions

How can I tell velvet from ich?

By the grain and the colour. Ich is separate white spots the size of a grain of sugar, easy to see in normal light. Velvet is a far finer film, gold, rust or grey, that usually needs a torch angled across the fish in a dark room to show up at all. Velvet also hits the gills first, so fast breathing and flashing often come days before anything is visible on the skin.

Can velvet go away on its own?

Very rarely. A single healthy fish in perfect water may fight off a light case, but velvet feeds on the gills and multiplies by the hundreds every few days, so an untreated tank usually loses most of its fish within two or three weeks. Treat the whole tank the day you see it.

Is copper safe for my fish?

At the dose on the label, measured with a copper test kit, it is safe for most scaled freshwater and marine fish and it is the treatment that works. It is not safe for shrimp, snails or other invertebrates at any effective dose, it harms live plants, and scaleless fish such as loaches and some catfish need half the dose or a different treatment. Move the invertebrates before the first dose.

How long does velvet treatment take?

Keep the tank dark and the copper level up for at least ten days after the last trace of dust is gone — usually two to three weeks in total at 28 °C, longer at normal temperatures. Stopping when the fish look better is the usual reason it comes back, because the cyst stage on the substrate is still hatching.