I have never used salt in a freshwater aquarium in 18 years of fishkeeping. I hear really good things about it, but have never used it. There are a few writes up on disolved minerals being benificial, but the amount of salt found in freshwater lakes, river etc. is very minimal. I am sure I am going to hear a large quantity of stories where people have used salt to treat all kinds of things. And im not saying it dident help, but as far as I understand, salt has very little positive effect. I know you take very good care of your fish and im really scratching my head here. But I would have gotten rid of the silver dollars, as anything they hav or had could easil be contracted to your new fish. I am sorry your going through this crap again.
One thing I and any seasoned hobbyist knows the best "cure" is prevention, and the best prevention is perfect water. I have seen your tanks and they are so well kept its insane. I really dont know what it could be. Your fish' immune systems have to be in the best shape with water quality like yours. Sorry I cant be much help. With the best of intentions, and in high hopes for you my friend.