"Dropsy" is often used by hobbyists to mean bacterial septicaemia with consequent "bloating", protruding scales, etc. in freshwater fishes. This is true in some cases, but is not always true.
For me, as a veterinarian, "dropsy" is a non-specific sign of fluid build up within the fish due to failure of fluid balance. Not all fishes with bacterial infections end up with "dropsy" and not all conditions presenting as "dropsy" are due to bacterial infections.
This fluid build up may be due several reasons including the failure to keep water out, the failure to excrete fluid or the over-production of fluid. To simply things a little, the fish maintains its fluid in balance by properly functioning skin/mucus barrier, gills, kidneys and cardiovascular system. If there is damage to any of these systems (e.g. skin ulcers from fungal infections in barramundi, bacterial gill disease in guppy, kidney cysts in goldfish, ovarian cancers in koi), then fluid may build up in the fish.
Thus it is essential to get a proper diagnosis of the condition so that you know what to treat for.
