I would look for signs of whitespot or velvet, any signs of flicking and rubbing against objects, or laboured breathing, any other signs of desease on the fish, like fluffy patches or being bloated.
Also have you added any new fish to the tank lately.
nope nothing new for ages...he has no other signs, he's eating fine and swimming fine...ages ago he did suffer from a very bad bout of columbris or whatever its called atm i cant remember sorry!, which wasnt down to me i took him in and nursed him back to health...could it be he banged it?
I don't think so, if water quality is good, which yours is it a sign of parasites or bacterial infection, just try abit of melafix for now, also it can be a sign of old age, plus bad diet.