Dropsy is a bacterial infection that can attack the internal organs. This bacteria seems to only affect fish weakened by bad water conditions, stress, etc. Excess body fluid builds up making the fish appear bloated and the scales protrude outward from the body. This disease needs to be caught early to have a good chance of successfully treating the fish.
Symptoms: Bloating, protruding scales, exhaustion, isolation, loss of appetite.
Treatment: There are commercial treatments available from any fish/pet store, but it must be diagnosed early.