Is he eating and defecating normally? I know it sounds yucky, but watching to see what your fish's feces looks like is a very helpful way of determining if there's a problem. Is his abdomen swollen or protruding at all? Is there any sort of sideways shimmy to his swimming motions? Does he appear like he's having any trouble keeping himself afloat?
A bit of the sideways shimmy you speak of, eats normally, the poo I haven't seen but I haven't had a chance to watch for more than 5 min. His colour seems a lot more faded compared to the other Harlequins too.
