It sounds like your fish has the parasite called Ick. This parasite is found in all aquariums. It normally cannot attach to the fish unless the fish is in poor health. You should test your water for high ammonia and nitrite levels. Your local store should have a medication available. Keep your aquarium lights off to disrupt the parasite life cycle. Increase your aeration and elevate your temperature to 85°F, at a rate of one degree every five hours until you reach 85°. Treat for 10 days. If you suspect your fish is ill, check out our guide on how to spot fish illnesses.