50% water change a week will keep your fish happy. Also don't forget to rinse or change your filter media as recommended. It sounds like you will have room for your new coming baby fish. You will probably lose some as the other fish may have them as a snack. Just think of it as population control. Or are you going to separate the mother and fry? You will have plenty of room especially if you keep up with the water changes. Water changes are important in that they give fresh water/oxygen to your fish. It is also helpful that you have an airstone to agitate the surface of the water which causes oxygen exchange which is beneficial. What type of Mollies and Swordtails?
