Dropsy or Bloat as they are sometimes called are terms that can be used for a couple of different diseases.  It may be attributed to such things as constipation, dropsy, or "Malawi Bloat", which at this point is not well understood.  It is a general term that encompasses a number of diseases that have the same gross external symptoms.  Anything that impairs or destroys kidney function can cause dropsy: bacterial, viral, or due to heavy metal poisoning or some other chemical toxin. Malawi Bloat- the direct causative agent in this disease is not yet known, but unlike true dropsy, organs themselves appear to swell and decrease in normal activity.  When this disease affects the kidney and/or liver, it is usually too late to affect any cure.  Improper diet has been implicated in the development of this disease.  A well balanced diet is the best mode of defense.  That and frequent water changes. True Dropsy- True dropsy is typically caused by a bacterial infection that affects the normal activity of organs such as the liver or kidneys.  This creates a buildup of fluids within the body cavity.  The organs do not enlarge in size and become damaged as the buildup of fluid creates pressure upon the organs.  Again, a varied diet and frequent water changes help to prevent this disease and if caught in the early stages, a treatment regime using Naladixic Acid or Triple Sulfa drugs that can be purchased at your local pet store will generally be effective. Swim Bladder Infection- The swim bladder spincter muscle can become infected.  This will result in either the swim bladder being permanently inflated or permanently deflated.  If the fish has a inflated infected swim bladder, it will not be able keep itself within the water column.  It will have a tendency to float to the surface any time that it stops swimming.  Conversely, if the swim bladder is deflated, the fish will be unable to keep itself off the bottom of the tank without considerable effort. This condition if caught very early in it's development, it can be successfully treated with Naladixic Acid or Triple Sulfa drugs that can be purchased at your local pet store.