What disease has my fish got?

Your fish has velvet it will be on the other fish too soon it spreds quickly take all yhe fish and put them somewhere clean the tank and evrything in it new gravle .There is a salt that cures it not table salt but sea salt.When you put them in it it makes them shed the slim coat and the velvet.then put them in the tank and a little salt in the tank not much say a table spoon for a 20 gal. tank.Belive it or not fresh water fish need a little salt