Your goldfish has Swim Bladder problems. This is not a Disease its a problem. Goldish with is problem have a floating problem either they are sinkers or floaters. The Fancy goldfish in general seem to have this problem. High levels of nitrates, not soaking there food and bad water quality can cause this problem. Also gulping too much air during feeding time cam cause this problem. I sujest buying sinking pellets for your goldfish.