Copper, Copper, Copper. If you truly have it, believe me its the best way. Been there done that. You will have to move fast, if your fish have it, velvet kills quick. Are there any other symptoms, rapid gilling, lethargy just hanging in 1 spot, not eating, cloudy eyes, etc?
Can you post a pic, with velvet your may not see any physical marking, diagnosis is made by the symtoms stated above.
Its tricky though, if its not velvet, you don't want to copper treat for no reason.