Well there can be so many possibilities that could have killed your mollies. You should first try to figure out what caused the fish to die so you do not make that same mistake again. We could be more helpful if you tell me your tank stats. Such as pH, ammonia, ammonium, water hardiness, nitrate, nitrite, etc. Also a 50% water change can be much of a risk. That is because with such a large water change you loose much beneficial bacteria that plays a role in keeping your tank balanced. The only way you can preserve beneficial bacteria from such a large water change is by soaking your filter sponge with the water you took away from the tank. Since the filter holds most of the beneficial bacteria that should help a lot. Also mollies need a bit of salt not a lot. They are brackish fish by the way. Not saltwater. Also it would be helpful if you can tell us more about your tank so we can hep you.