in the wild, mollies are mostly bw fish. but since most of the ones you buy in the lfs are tank bred, and usually in fw, fw is fine. they are indeed tolerant of salt and can adapt to it easily, but it is not needed to have healthy mollies. i keep mine purely fw. some people like to add a little salt anyway, if they don't have non-salt tolerant tank mates that is. but again, its not vital.