How long has it been liek that? I just euthanised an angelfish with this condition, It could be alot of things but i concluded it was a swim bladder infection, this could be caused by spoiled food, or simplya result of old age, much Alzheimer's or cancer in humans, If it is a becterial infection you could try some maracyn and see if it comes around, if its too late for the fish it may have permanent swim bladder damage which would mean there is nothing that can be done.