Stop driving the pH of your water down. The acids required to do that will add to the mineral content of your water in a way that mollies will not appreciate, much less the rest of your fish. My own mollies thrive and reproduce in quite hard water with a pH of 7.8.
Instead, try doing a large water change just in case you have something in your tank that we do not measure that is harming the fish. Pet shop mollies are not always the best choice if you have relatively soft water. If you find it easy to buffer the pH of your water, it is far too soft for typical pet shop mollies. If it is a major struggle to reduce the pH and then hold it, you may have enough mineral content in the water for mollies to thrive. Let us assume the latter is your situation. That means that simply replacing most of your water with nice dechlorinated tap water is likely to really perk up your mollies. I have been known to replace over 90% of my water with tap water when I suspected there might be something wrong in one of my tanks. The fish almost always looked much better immediately.
