Dropsy usually indicates either kidney failure, or a bacterial infection if I remember freshwater fish bio correctly. Kidney failure isn't really treatable, but bacterial infections are.
For the bettas you lost, do you know how old they were? How often did you do water changes (and how large were they)? Did you feed lots of food, or was there enough food that it could reasonably decompose in the tank?
So many fish/plants/inverts to keep, not enough aquaria.