Velvet can look like a thin "velvet" skin is covering the fish or appear as a light dusting of gold spots, either way its a nasty disease that mainly effects scaleless species such as catfish and loaches.
To treat it effectively the tank needs to be completely covered to stop any light from getting to the water, the planktonic larvae of the parasite are photosynthetic drawing the energy they need to grow from light so if there are any plants in the tank they will need to be removed first. Also you need to raise the temperature to 28c to speed up the life cycle of the parasite which will lower the oxygen levels in the tank considerably so adding a large airstone to help aerate the water is recomended. Protozin by Waterlife aquarium products is about the best medication available for treating velvet and should be used along with the above methods.
