there are two forms of dropsy: chronic and acute. acute is what most people think of, where the scales stand up because hte abdomen swells suddenly.
mrparker, it may be chronic dropsy, which is a slow swelling from cancer in internal organs.
try firebug's remedy to constipation, and see if that makes the swelling go down. have you whitnessed him passing feces? is he eating normally? how's his swimming pattern?
let's hope it's just constipation!