Dropsy is a bacterial infection of the gut and usually comes on pretty quickly.....over the period of a day or two. The tell tale sign of dropsy is, if you look at the fish from the top, their scales will stick out like a mini pine cone.
Dropsy is carable if caught in time. If the fish is valuable, it might be worth getting to the vets. Otherwise, Octozin is recommended
http://www.waterlife.co.uk/waterlife/octozin.h tm
Chances are that if your fish has been getting "dropsy" all summer, than it's probably a nice fat koi.
Why not register at
http://www.koichat.com/ and ask the real experts
Good luck