:( If he has severe parasites, then yes, it can cause dropsy. This is tricky because I really want to help the dropsy but I'm afraid if we ignore his tail, the rot will take over. Okay. Two options and unfortunately, both are going to cost money. If you can't do this, that's perfectly understandable.
First is a conservative method. You'll need Kordon Methylene Blue for this. One tsp per 5 gallons and he'll need to be bathed in that for 30 minutes twice a day.
Second is Seachem Kanaplex, API Furan-2, and Seachem Metro in combination. Both Kanaplex and Metro would need to be ordered online. This is an extremely strong concoction that would treat bacterial and parasitic symptoms. The question, if you wanted to do this, is if Marley could hold on long enough for the meds to arrive.
Finally, continuing with the course you're on now would be fine too. To be honest, dropsy symptoms often (not always but often) signify that the organs are beginning to shut down and the end is near. If you don't want to spend money on a fish who may die anyway, that is understandable.
I wish I had more positive news for you. :(