Actually, maybe I should say that I have two goldfishes. One lionheaded and one silver, much smaller than the other one. The little one doesn't seem to have any symptoms, only that he is keeping compay to the big one, on the bottom of the tank (which I had also observed as something wrong, they are usually very active fish)

I don't know what as happened to your fish but the same has happened to mine, you should put the sick fish in another tank