Velvet is caused by a microscopic parasite that anchors itself in the fish's skin and gills and feeds there, and it earns its name honestly: a coating of countless tiny parasites gives the body a soft, dusty sheen, as though the fish were cut from velvet cloth. It is one of the most often missed fish problems for a simple reason — in ordinary tank lighting, on many fish, the dusting is nearly invisible until the infestation is well advanced.
The dusting, and how to look for it
The coating is gold, yellow-brown or rusty grey, and fine — far finer than the distinct white grains of white spot. On dark fish it may show as a dull golden bloom; on pale fish it can hide almost completely. The reliable way to see it is to darken the room and shine a torch across the fish from the side: the beam raking along the body lights up the dusty texture that flat overhead light conceals. Keepers who add this torch check to their routine when any fish seems vaguely unwell catch velvet days earlier than those who rely on the tank light. Look at the whole body, but pay attention to the area behind the head and along the back where the coating often shows first.
The behaviour that gives it away
Long before the dusting is obvious, the fish's behaviour complains. Fins are clamped tight to the body rather than displayed. The fish rubs or flicks itself against gravel, rocks and decor — flashing — trying to dislodge an irritation it cannot reach. Appetite fades. Most tellingly, breathing quickens: velvet attacks the gills as well as the skin, and a fish breathing fast at rest, or gathering near the filter outflow or surface where oxygen is richest, is describing gill trouble. In advanced cases the skin can produce excess slime and begin to peel in patches, and small fish can decline very quickly.
Why it spreads through a tank so fast
The parasite's life cycle includes a free-swimming stage that leaves the fish, divides in the water and on surfaces, and seeks new hosts — so one dusted fish in a community tank is a tank-level event, not an individual one. New arrivals are the usual introduction, which is why velvet so often appears a week or two after a stocking change, and why quarantine earns its keep. Because the gill damage can kill before the skin signs become dramatic, the speed of the response matters more with velvet than with many slower problems.
Acting on a suspicion
If the torch test shows a dusting and the behaviour fits, do not wait for certainty to arrive on its own: isolate the affected fish if you can, and get advice from an aquatic veterinarian or fish health specialist promptly, describing the dusting, the breathing rate and which other fish share the water. Avoid passing nets and equipment between tanks without cleaning them, since the free-swimming stage travels on wet kit. Note the date each fish first showed signs, and what you observed, in that fish's Pet Passport on KinsLe — in a spreading outbreak, that dated sequence is how you and the specialist work out where it started and whether it is still moving.