Dropsy is basically kidney failure in a fish. It is actually a symptom of a disease rather than the disease itself. It can be caused by a virus, a bacteria, a parasite,or adverse water conditions. Predisposing factors include stress, poor nutrition, genetic weakness due to inbreeding, or senility. The primary route of the infection is through the gut and usually caused by foraging on the floor of the aquarium or through cannibalism of another infected fish. There are also known to be fish who exhibit no symptoms of the disease while being carriers of the disease and can pass the disease along through the infected waste they deposit on the floor of the aquarium.
Prevention is the key to this disease as cures are not readily available or impossible. Maintain the best possible conditions in the aquarium and isolate any infected fish. Avoid all stress to the fish whether infected or not.
Once a fish is isolated fish may respond to a antibacterial bath for at least 7 days. If the fish does not respond, antibacterial food containing Oxytetracycline or Chloramphenicol for 10 days or Sulphamerazine for 3 days may be effective.
I lost a Betta to Dropsy and it is not a disease that is pleasant. I am so sorry that your little Sirius went through that.
Rose