From American Aquarium Products: An explanation on disease prevention. Be prepared for a detailed lesson! This website is amazing for its thorough information. Be patient, and go through it all. You can cure your sick aquatic pet!
Dropsy is generally a symptom of something else other than the classic and more noticeable symptoms that are then labeled as "Dropsy". Most often Dropsy is Kidney related, which results in swelling and fluid retention due to poor kidney function results in the classic "pinecone" look of fish sick with Dropsy. Poor osmoregulation is usually the second most common cause, and in more rare instances digestive, and maybe liver malfunction/infections.