Mori, there are literally hundreds of published articles available regarding the physiology of goldfish, each testifying to the fact that they have long, winding intestines, but literally no actual "stomach." This is also why they are so prone to impactions from not enough fiber and vegetable matter in their diet.
Ikuzo, test your water parameters and make sure they're intact. Also, examine his diet and determine if the food could be a factor. Many times what one might think is a swim-bladder disorder is actually a blocked intestine, and this, too, can be fatal. Try feeding some shelled, cooked peas for a few days and see if this helps, along with a few days of increased water changes and vacs. Hopefully this will perk him up some.
Also, I am by no means a fish expert, trust me. I am an RN, and it's really not a big leap from diseases of the human body to diseases of fish. Bacteria, viruses and fungus all react pretty much the same way, regardless of their host.
