Okay the other not very nice alternative is an infection of the swim bladder, If its only one fish affected. The fish uses its swim bladder to swim and position its self in the tank. if the swim bladder gets damaged/infected the fish loses its ability to swim straight, Normally recognised by the fish swimming at a funny angle, some times belly up or weaving up or down eratically. If this is the case there is no cure unfortunatly and the only option is to put it out of its misery.