Velvet disease, also called gold dust disease, is a tropical fish disease caused by dinoflagellate parasites of the genus Piscinoodinium, which gives the fish a dusty, slimy look. The disease occurs most commonly in both tropical and (to a lesser extent) marine aquaria.
Initially, infected fish are known to "flash", or sporadically dart from one end of an aquarium to another, scratching against objects in order to relieve their discomfort. They will also "clamp" their fins very close to their body, and exhibit lethargy. If untreated, a 'dusting' of particles (which are in fact the parasites) will be seen all over the infected fish, ranging in color from brown to gold to green. In the most advanced stages, fish will have difficulty with respiration, will often refuse food, and will eventually die of hypoxia due to necrosis of their gill tissue.
A roundup of what guppy hobbyists do to treat velvet
Velvet does not seem to be very prevalent but it does occur and here are things that will help you manage it. You will of course need to use medication. Some brands for treating guppies with velvet include American Products, Kordon and Mardel now Fritz Aquatics. Essentially you have over half dozen products to pick from on the market and either way they will cause temporary discoloring of the aquarium. The color will either be blue (Methylene Blue) or green (Malachite Green) essentially. And you can dye your fingers when handling it, much like ink. Anyway some of the brand names include QuickCure, Acraflavine, Methylene Blue and, Mardel Coppersafe, and Copper-AID.
Velvet disease ("rust disease") is highly contagious and it has a hazy white appearance on the body. Ich and velvet are sometimes hard to tell apart. It may be difficult to see until a heavy infestation develops. Bleeding can eventually occur, with the skin peeling off. When the guppy's gills are affected the fish will have difficulty breathing and may die from asphyxiation. Read more.
Velvet is caused by a small yellowish organism called Oodinium linineticum. It has whip-like flagellum to propel itself and another in a constriction in its middle. On small fry velvet is particularly serious. The parasites have root-like tentacles which penetrate the fish's skin and through which they draw nourishment from the body. Read more.
Treatment: (1) Acriflavine. (2) Methylene blue diluted as much as one-hundredth of a grain in a gallon of water, killed velvet, provided the temperature has been maintained from 77 to 86 degrees.