Add baking soda or salt for the mollies!?!? The more the water is changed from your tap conditions the more dangerous every water change will be, because the water will have to match the tank water every time. Also, mollies DO NOT REQUIRE SALT! Mollies can survive in salty conditions, but they do not require it. They do, however, require HARD water.
The easier (and safer thing) to do would be to put some crushed coral into the tank. The crushed coral will slowly dissolve and slowly increase the hardness. (Adding baking soda will also increase the hardness, but extremely fast - far too fast for most fish to survive). Instead the crushed coral will continually dissolve. The lower the pH, the faster it dissolves. This will also raise the pH as it does so. When the pH is higher though it won't dissolve as fast. As far as continual maintenance goes, you just do more frequent, smaller volume water changes. When you notice the amount of the crushed coral decreasing, just add enough to make it the same as the original amount in the tank.