Dropsy is generally a secondary infection - i.e, it was caused by something else, possibly a parasite, virus or bacterial infection.
Success rate with treating dropsy is unfortunately very low (although I have had limited success in the past). I isolated the fish, increased salinity of the water greatly, and increased aeration and temperature. This has unfortunately only worked for me once, so I'd suggest removing the tetra and looking for a possible cause. Checking your water parameters would be first port of call.