I would QT her, since she was the new fish that most likely came from poor water conditions (that you had no control over) and stressed and then with the added stress of a new and clean tank, she could have had a bit of shock, low immune response etc.....
If her scales are sticking up and then it is most likely dropsy and recovery is rare once that symptom show up and can be a sign of system shut down.
I would try Epsom salt 1tsp/gal with 100% daily water changes for at least 10 days, hold food for 3 days and then feed half of normal feeding every-other day, the Epsom salt can help with the edema to relieve the symptom but it won't reverse the damage, you can also go the antibiotic route with broad spectrum to cover both gram neg and pos, I don't use OTC medications so I can't recommend any.
Frogipoi:
IMO/E-15 females in a 20g tank is just fine with proper water changes like this poster is doing.