It's a disease in all freshwater fish...maybe saltwater too, but I don't know them. The very best treatment that I've ever used is Medi-Gold from here https://hubpages.com/animals/Goldfishconnectioncom... combined with Epsom salt (about 1/4 teaspoon per 10G). If your fish is already sick then you don't have time to order medicated food, that's something that has to be on-hand ahead of time, and in that case try to find some Maracyn-Two (available from some pet stores) and treat with that -- though it tends to be considerably less effective than medicated food. Make sure your water parameters are in great shape, and remove the carbon from your filter throughout the treatment process. For goldfish, while they're tolerant of varying nitrate levels, I recommend keeping it under 20ppm throughout treatment and, even with healthy fish, never letting it exceed 40ppm. Bear in mind that dropsy is almost always fatal, especially if you're not prepared to treat immediately once it's detected, but with some preparation and prevention (in the form of careful water monitoring and quarantining new fish) it can be dealt with.