One of our club members is a veterinarian and did a surgery at a meeting for a goldfish with a swim bladder problem. It seems that the swim bladder is connected to the digestive tract of fish that have one and is filled with air to make the fish neutrally buoyant. In this case the fish had an over sized swim bladder and it was causing the poor fish to float upside down. That meant it could not right itself and swim normally. He had been treated by literally deflating the swim bladder with a hypodermic needle but it would return a day or two later and he could again not right himself. After the surgery, the fish had a radically reduced swim bladder size and it was not enough for him to float with neutral buoyancy. Instead, he would sink to the bottom when not actively swimming. The cause of the over sized bladder was thought to be that the fish was overfed when young and developed a larger than usual digestive tract, which of course included an over sized swim bladder. It is a progressive chronic disease in fish, not something that will strike overnight. I see the diagnosis of swim bladder all too often when a problem develops that is difficult to diagnose but do not consider that the most probable cause in most cases.