can you see if its a battle wound? what're the water parameters? add salt and up the temperature, if it's nothing serious should heal up soon. if its caused by bacterial infection then you might have to use med (ie. melafix, try to get the professional strength one, 64oz, 5ml treats 50g intead of the 5ml for 10g ones. this way you won't waste money on it) if salt alone doesn't improve it.
here's a link for information on salt and dosage.