Kitten, I don't want to get into a row with Tokis-Phoenix but I have plenty of very healthy mollies with no salt added to my tanks. The main thing that mollies need is a reasonably hard water and a pH well over 7.0. A marine salt will give you both because marine tanks run with a pH over 8 and of course with lots of dissolved solids. In my case, the water from my tap comes in at 250 to 325 ppm TDS depending on the time of year and with a pH of 7.8. I never add salt for livebearers, including mollies, because they really don't need it. They do need lots of dissolved solids and a decent pH. All of that said, mollies can thrive in straight saltwater like you might use in a reef tank so it won't hurt them if you have some salt. The Plec and tetras in my community tank also see high solids and high pH water and are doing just fine. I have no experience with rainbows to say what they might need. The picture earlier in this thread is a molly in a salt free tank with high TDS and 7.8 pH.