It does sound like dropsy. One treatment for it that I've read is increase temp to 82-86F (increase aeration), add 1/8 tsp of epsom salt (Magnesium sulfate
heptahydrate) per 5 gallons of water. Treat for about 2 weeks! This may not work for you if you have acute dropsy, which is what it sounds like you are dealing with.
Another site that has some good info on Dropsy is
HERE. Acute dropsy has rapid onset and is difficult to treat because it can be caused by a number of factors. Supposedly it is not contagious, and it may be that conditions in your tank are effecting all the fish the same way. However, if it is parasitic in origin, you need to treat all the fish for parasites first. The problem is
which parasite are you dealing with? One
study (1986) of dropsy in Catla catla (an India carp food fish) found it was a combination of myxosporidian parasite and a strain of
Aeromonas hydrophila. Maracyn II is a possible treatment.
Another
study I found iseems to say that formalin is an effective treatment. Both of these studies were done back in the late '80's.
There is also a high risk of dropsy occurring as a result of septicemia. Septicemia needs to be treated immediately or it is fatal. Did you notice any redness on the affected fish? Dark red near the caudal fins or abdominal or head areas?
Meanwhile, what's your current situation?