You do realize that dropsy is not a disease, right? Dropsy is simply the name given to describe a set of symptoms that indicate a number of actual diseases and conditions. Thinking of it as a disease is sort of like calling a fever a disease...when it's actually just a general symptom that can be caused by many factors.
Is this fish in that tank you have shown in other posts, with a large number of large koi and goldfish bumping into each other in cloudy water? If so, I think we may have discovered the cause of the symptoms.
None of those fish require a heater in their tank. If you decide to add one, be aware that raising the temperature of that tank will decrease the level of dissolved oxygen, possibly stressing all the fish even further.
Clean water, bigger water changes done more often, remembering to plug the filter in, all will help those fish out.