Black Moor is the bulgy eyed black fish I take it? Those are coldwater fish, not generally compatible with Platys so you might want to consider taking one back.
What size tank? Do you have test kits to measure ammonia, nitrIte, nitrAte and pH? How long has the aquarium been set up? What kind of substrate (gravel, sand, rocks, etc) and decorations do you have? What kind of filter? What water temperature?