Velvet disease in fish: how to spot gold dust disease and treat it fast

Velvet (also called gold dust or rust disease) is a parasite that coats fish in a very fine gold or rusty dust and attacks their gills, and it is more dangerous than ich. The most important thing to do is add aeration now and start a full course of treatment, usually copper, today.

In freshwater fish it's caused by Piscinoodinium pillulare (older books call it Oodinium), a dinoflagellate: a single-celled organism related to algae that, like algae, can use light for energy. It attaches with tiny root-like threads and damages skin and gills, often starting on the gills, so a fish can struggle to breathe before you see anything. The Merck Veterinary Manual says high death rates are common, and small and young fish die first.

How to identify it

A very fine gold, yellow, rust or whitish dust on the body and fins. It's much finer than ich: more like powder than salt grains.
Flashing (rubbing on objects), clamped fins, and a fish that seems "off".
Faster breathing than the other fish.

The flashlight test: turn the room lights off and shine a flashlight at the fish from the side, at an angle. Velvet often shows up as a golden sheen you can't see in normal light.

Easily confused with:

Ich: larger, distinct white grains.
Gill flukes: fast breathing and flashing, but no dust.
Low oxygen or ammonia poisoning: gasping with no dust at all. Test the water to rule these out.

A microscope look at a gill or skin sample is the only way to be sure.

When it's a problem

Early: fine dust visible with the flashlight, some flashing, slightly faster breathing. Treat today.
Serious: fish gasping at the surface or crowding near the filter outlet; loss of appetite, lethargy, skin peeling or looking cloudy, small red spots.
Emergency: sudden deaths, often several in a few days, small fish first. Velvet can wipe out a tank of small fish within days.
Normal, don't worry: a fish that breathes as calmly as its tankmates and shows no dust in the flashlight test. Velvet kills fast, so if you do see dust, act.

What to do right now

Do the flashlight test on every fish. Assume the whole tank is exposed.
Add aeration straight away: an air stone, or point the filter outlet at the surface. Velvet attacks gills, so oxygen buys time.
Turn the tank lights off and keep the tank dim. Darkness alone won't cure velvet, but the parasite uses light, and dimming is low-risk support.
Test ammonia, nitrite and pH, and do a 25–30% water change with temperature-matched, dechlorinated water.
Remove activated carbon from the filter, or it will soak up the medicine.
Move shrimp and snails out if you can. Copper, the main velvet treatment, kills them.
Start treatment today.

What to do next

How the life cycle shapes treatment

Velvet's life cycle looks a lot like ich's. The feeding stage drops off the fish, forms a cyst on the bottom, and divides into many free-swimming spores that look for a new host. According to Adrian Tappin's review for ANGFA, the full cycle takes about 10–14 days at 22–25°C (72–77°F), slower in cooler water. The cyst stage resists chemicals; the free-swimming spores are the easiest stage to kill.

Copper

Copper is the standard treatment. Both the vet manual and the ANGFA review list it. It works on the free-swimming spores, so it has to stay in the water through at least one full life cycle. Typical courses run about two weeks; follow the label.

Copper has a narrow safety margin. Too little doesn't work; a little too much kills fish. Use a copper test kit and test daily.
Copper is far more toxic in soft water. Don't use it if your total alkalinity (KH) is very low (under 50 mg/L, or about 3 dKH) unless a vet or the product label says it's safe (UF/IFAS).
Copper sticks to gravel, rock and silicone, and can leach back out later. A tank that has had copper in it may never be safe for shrimp again.
The ANGFA review lists some fish as copper-sensitive at very low doses, and says copper is best used by experienced keepers.

Other options

Chloroquine. The Merck manual lists it for ornamental (never food) fish, and the pet owner version says a vet may recommend it. It's prescription-only in many places.
Salt. Short, strong salt dips have been used for salt-tolerant species, but fish can be reinfected when they go back to the main tank. Low-level salt in the tank is not a reliable cure.
Heat. Raising the temperature speeds up the life cycle so spores meet the medicine sooner. Very high heat has been reported to control velvet, but most fish can't tolerate those temperatures safely. Don't go above what your species can handle, and raise it slowly with extra air.

Ich-and-velvet combination products vary a lot. Check the active ingredient. Follow the label, and measure your real water volume.

Cautions

Shrimp, snails, crabs: copper kills them, often at levels too low to measure with a hobby kit. Remove them before treating.
Plecos, other catfish, loaches and scaleless fish: often more sensitive to copper. Many labels advise a reduced dose. Watch them closely.
Live plants: some don't tolerate copper.
Filter bacteria: test ammonia and nitrite every day or two during treatment. Sick fish plus medicine plus a stressed filter is a bad mix.

How long to treat

Keep treating for the full course, even after the dust disappears. The cysts on the bottom are protected and keep releasing spores. Because the life cycle is slower than ich's, velvet treatment usually runs longer. The pet owner version of the Merck manual says a recheck in 7–10 days may be needed.

Products that help

Copper-based medication: the standard velvet treatment, for tanks without shrimp or snails and with alkalinity above 50 mg/L.
Copper test kit: test daily during treatment to stay in the narrow safe range.
Alkalinity (KH) test kit: check before using copper, because soft water makes copper far more toxic.
Ich-and-velvet combination medication: an option, but check the active ingredient before you buy.
Flashlight: for the side-on test that shows velvet's golden sheen.
Air pump and air stone: extra oxygen for damaged gills.
Liquid test kit for ammonia and nitrite: to watch your filter during treatment.
Water conditioner that removes chlorine: for safe water changes.

Keep on hand before trouble starts: a copper medication, a copper test kit, an air stone and a flashlight. Velvet moves too fast to wait for a shop to open.

How to prevent it

Quarantine new fish for at least 4 weeks in a separate tank. Check them with a flashlight every few days.
Keep water clean and stable: weekly tests, regular water changes, a reliable heater.
Don't overcrowd. Parasites spread fastest in packed tanks.
Give each tank its own net, or disinfect and dry nets between tanks.

Why it happened

A new fish or plant brought it in without quarantine.
Stress tipped a low-level infection into an outbreak: poor water, crowding, or a sudden temperature drop.
Shared nets and hoses. The ANGFA review even suggests spores may travel in water droplets between closely placed tanks.

When to get help

Velvet kills fast, so ask early. If you have rare or valuable fish, if you aren't confident with copper, or if fish are still dying after 2–3 days of treatment, call a vet who sees fish. They can confirm the diagnosis under a microscope and prescribe chloroquine where it's legal. A good local fish store can help you choose a copper product and test kit. Be honest about the limits: fish with badly damaged gills may die even after the parasite is gone.

In the game

In Multiple Tank Syndrome, the Velvet card is "a gold-dust parasite that kills small fish first": every fish 2 inches or smaller in the hit tank dies. An Ich and velvet medicine card or the Medicine basket saves the tank. Real velvet hits small fish hardest too, which is why tanks of tetras and rasboras are so vulnerable.