Dropsy is the result of an underlying illness. It is not an illness in itself. It is caused by something making the fish retain fluid and eventually will lead to kidney failure. It can be caused by dirty water which as a result gives the fish a bacterial infection that leads to dropsy, but it can also be caused by an infestation of parasites, bacterial in live or frozen foods, an after effect from an injury, and so forth. Once the pineconing starts, dropsy is almost always fatal.
Chances are, your fish is constipated. They need to be fasted at least one day a week, and have a blanched pea one day a week to keep things moving along. A variety of frozen foods like daphina, bloodworms and brine shirmp will also help if you are only feeding pellets and freeze dried foods. You should also clean up any uneaten food on the bottom of the tank immediately after they eat. The food can go toxic in a short time and make your fish ill. A new turkey baster works wonders for this.