I would QT and start with Epsom salt 1tsp/gal along with 100% daily water changes for 10 days-I would keep my temp in the low range of normal (76-78F) to discourage bacterial growth and avoid flex with any sudden high temp with an immune compromised fish. Most fish will stop eating when sick-so he most likely started to get sick when he stopped eating and he just didn't have any outward symptoms yet. I would not feed anything yet-hold food for 3-4 days and then offer half feeding during treatment
At this point I would gear my treatment toward the bloat/dropsy with Epsom salt not aquarium salt-the clean water as well as the Epsom salt should also help with the fin issues, hopefully he is not in system shut down yet....how old is he or how long have you had him.
I am assuming this is a filtered, cycled tank with at least 3-4 50% weekly water changes in a 20L tank..that is a little over 5gal right....with 5 Bettas???? and no long term use of aquarium salt...correct????