Sounds reasonable to me. Isolating black mollies away from more aggressive fish (like tangs) is always a good idea.Here’s another question. Locally my use of mollies has been getting noticed and I had someone ask me this past weekend what I’d do with his situation: A guy with a frag table (8x4) and about 5 tangs in it. Says he’s never seen them with ich or shown signs of ich, but doesn’t want to pull them out to QT them for stress and algae reasons. I suggested he acclimate some mollies and put them in the sump (not sure how tangs would tolerate mollies?), if they don’t show anything after 4 weeks, consider the tank clean for ich.