That is interesting and so I had some doubts, I wonder how the mollys would react to different diseases like velvet or flukes, what would the symptoms be or would it just go unnoticed?The are different species of mollies and some adapt better than others (sail fins do better than sphenops). All require a slow acclimation, especially if going from pure FW to full salt, I prefer doing that in a tank over three days.
Using mollies as a disease indicator really only works well for marine ich.
Jay