swim bladder disease may be caused by many different factors including virus, bacteria or diet:
a virus can attack the epithelium of the swim bladder causing inflammation which makes the tissue too thick for gases to diffuse into the blood system. when this happens the fish exhibits lack of buoyancy because gases have nowhere to go. unfortunately, if this is your problem, it's untreateble.
bacterial infections can cause the same kind of thickening of the swim bladder epithelium as viruses.
anatomy ... globoid-shaped fish like ornamental goldfish are predisposed to problems with the swim bladder because of their anatomy which result in mis-placement of the bladder in the body cavity. this can lead to food impactions, which in turn clog up the pneumocystic duct.
feeding dry foods which tend to take on water like a sponge and expand in the fish can result in food impactions. due to the swelling/expansion of food in the gut.