I tried looking it up and apparently "dropsy"
but it's not.
Anyways, intestinal infection, may be a cause... Epsom salt may help.
In that case immediately remove him. Actually I suggest removing him immediately anyways just in case, especially if he dies and the others become ill from him dying.
Or, it could be bloat, which being ravinous eaters they may gulp air (like goldies too)....
Another thought, what do they get food? Food that is rich in certain things can cause problems..