that is mostly a sign of a common disease known as ick [ tiny white spots on the body if not treated soon fish will die occasionally. ] treatment for this is ick away, ick guard, or something that has to do along the lines of that. if you need help ask someone at the pet store the might be able to tell you. but however there is other common disease that can cause that. im not sure what can but the only one that i know of is ick.