Tank water etc is all fine, its a 4 ft tank, i do 2 water changes a week, so that is not the prob. He isn't getting harrassed by other fish in the tank as he is the peace keeper, anyone starts anything he goes and seperates them. Tank has been setup for well over a year and i've had him the longest. I noticed today he is fine, swimming as normal, like nothing has happened. Maybe he bumped his head as someone said. I guess now i'll never know. Google wasn't much help. Thanks everyone. I'll just have to keep an eye on him.