Dropsy can be caused by many things. It can even be localized in one spot of the body and not an all over bloat. It is a symptom of an internal infection and organs shutting down, most often the kidneys. When organs start to malfunction, fluid builds up in the body and pushes the scales outward, causing that pinecone effect. Dropsy can even be present in an old fish who, aside from reaching the end of his natural lifespan, is otherwise healthy.
The thing with parasites is this: all fish have a certain amount of parasites in and on their bodies. A healthy fish is strong enough to keep these parasites from becoming an infestation but a stressed fish with a compromised immune system isn't strong enough to fight them off and that's when they become a problem. Since this guy was kept in rotten water conditions, it makes sense that he wouldn't be strong enough to fight anything off.
I still recommend 3 tsps of epsom salt per gallon to help keep the swelling down. If you have any Indian Almond Leaf or naturally dried oak leaves, I recommend those as a tannin source. Medication is optional but if you do decide to medicate I would suggest either API General Cure or API Furan-2.