it could be severe pH shock... if these bettas are bred and grown in pH below 7.4, then they may not adjust. are you acclimating your fish to the pH, in addition to floating the bag?
i only suggest this because rasboras and plecs may be more resistant to pH 8.4 (remember, thats ten times more alkaline than 7.4). otos are generally softer water fish, but i've found them to be tough little suckers.
pH damage to fish is VERY harsh. if it throws their natural blood chemistry out of whack, the death can be pretty long and drawn out. organs become maxed until death, which may account for some swelling. an ruined swim bladder would explain the dropsy symptoms.
next time you give it a go-round, i'd check the water it comes in...
and $0.98's your change.