I am fairly new to the fish world, having only bettas and smaller fish in the past. a buddy of mine gave me a 55 gal fish tank with a bunch of tropical fish. the biggest one is a gold severum who seems to have discoloration on his body. one side is worse than the other, with grey bruise like marks along his scales. he doesn't seem sick, swims, eats and still comes runnin' at dinner time. none of the other fish have these marks on them and all fish get along just fine. does anyone know what is wrong with him?