There is no actual disease called Dropsy,
it is a clinical description of the condition which is often a secondary infection. also known as Pine Cone disease the fish's body swells up, eyes can bulge out and scales are raised.
Dropsy is extremely serious and difficult to treat, the swelling is caused by excess fluid. remove the infected fish to a suitable quarantine tank and add salt at half oz per gallon, I would also recommend the addition Sabbactisun anti bacterial treatment.
Under Veterinary prescription you could use a suitable antibiotic, however recovery from this condition is rare as internal organs are often damaged.