Yeah, sounds like constipation problems as one of my goldfish did the same thing. Move it to a small bucket with a gentle airstone and put in a couple of teaspoons of epson salt (cures constpation), feed it peas for the day and it might be OK. Mine had this treatment and fully recovered with the day, he's now happily out in the pond getting conditioned for spring. Also, tell them not to feed pellets, this is what normally causes the problem, flakes are good. Goodluck