Make sure it is dropsy, and not constipation. If the swelling is the stomach only, it might not be dropsy. If you notice the body behind the stomach swelling, then it is probably dropsy. Look straight down on the fish to observe swelling, not from the side. Salt doesn't do anything for dropsy. Epsom salts does help, if you catch it in time. If it is caused by internal damage from an accident or fight, then it is not contagious. If the damage is from the kidneys shutting down due to bacteria, then it is very contagious. Personally, I cull a dropsy fish immediately, no matter how much I like it. It's not worth what it can do to a fish room. Some have said they cured the dropsy, only to find out it affected the fish in other ways. I was even told it made their Bettas sterile. I just cull it immediately, and move on to other fish. You can try meds for dropsy, and a epsom salts bath. If you don't notice immediate results, then it will probably not survive.