Preventing goldfish dropsy is much easier than curing it. The most prominent cause is from bacteria that’s introduced either by the water you use, contaminated food, or simply water that hasn’t been cleaned/filtered properly. Having a good aquarium filter in with your fish is probably the best way for you to prevent contamination from happening in the first place. Prevention should work in almost 80% of all fish: Prevention isn’t always possible, since goldfish dropsy can also be cause by kidney, heart or liver disease in the fish which may be cause by natural, or unnatural causes.
There are goldfish dropsy water treatments that can be added that may or may not help depending on the fish’s circumstance and how far the dropsy has progressed. Aside from any external medications you give them; isolate the infected fish from others right away. For the most part it seems that dropsy isn’t contagious, but the added stress of other fish pushing it around and nibbling at it may push the fish over the edge and cause death.
Let Nature Take it’s Course
In the end, it will be up to nature and the fish as to whether it will survive the dropsy or not. Let nature take it’s course: Don’t handle the affected patient and let them rest. Of course you’ll undoubtedly want to keep an eye on them, but avoid poking them or scaring them at all costs.