Sounds like ich. Immediately raise the temp to 90 - 95. If you have aquarium salt, add 1Tbsp per 5 gallons. The temp will kill the ich in a few days, and the salt will help him fight the disease. If it's just ich , it is in a very advanced stage and he may not be save-able. Piranha are a warm water fish and are good into the low 100's so don't worry about the temp. Don't add an ich medicine if he's in this advanced of a stage. They are toxic and will more than likely finish the fish off for sure.
How old is your tank, and what are the water parameters, ammonia, nitrite, nitrate, temp, and what is your water change schedule?
It's a skin disease called ichthyophthirius multifilis or ich for short. You can buy drops to put into the water of your tank to treat this.
http ://www.fosterandsmithaquatics.com/pr ... this is where you can buy the drops or go to your local pet store
it sounds like ich. do the spots look like grains of salt? if so, you should treat with some liquid aquarium antibiotic, like rid ich, or coppersafe. treat the whole tank, because ich is very contagious and will spread very fast.
NOOO !!!!!!!! NEVER RAISE TEMPERATURE TO 90. WOW! CR8ZY PEOPLE!!!! Raise temperature to 82-84 at the VERY MOST. Also buy ich treatment.
