Mollies like brackish water. I add ocean salt to my mollie tank, and they're MUCH happier with the salt than without. I add 2 heaping tablespoons per 10 gallons, and every fourth water change (25%) I add another 2 tablespoons.
Not scientific, but my mollies are happier than ever and I believe I'm about to have fry. I'll try to find this thread and post the results of that.