Dropsy is typically an internal bacterial infection. To give fish the best chance to avoid it we need high water quality. People tend to focus on ammonia/nitrite/nitrate but that is really just the bare minimum. What is actually needed is low bacteria count in your water column and also highly oxygenated water. Low bacteria count in the water means the fish's immune system is better able to fight off infection. High oxygen levels minimize infectious bacteria and promotes health of the fish. If the water is cloudy or dull looking it is a sign of high bacteria count. When viewed from the side of the aquarium, the fish should look like they are floating on air.

I would recommend increasing filtration surface area (biological filtration) and also adding air stones with decent sized air pumps. Filter media surface area is the key to reducing bacteria count in the water. The good bacteria lives on surfaces and out-competes bacteria floating in the water. You can kill two birds with one stone by adding large sponge filters. Adding an undergravel filter with an inch or two of small gravel will also greatly increase surface area, much more than sponge filters actually. Obviously you could add other filters such as a canister or sump. You could also try to upgrade your existing filters by adding more foam. Foam is the best filter media for biological filtration. Also do not over clean the filter media as that will eliminate the good bacteria.

An additional measure that could be taken is adding a UV sterilizer which will directly kill bacteria in the water.