Mollies are a species of freshwater fish that prefer hard water. Hard water contains an increased amount of dissolved minerals, such as calcium and magnesium, which provide mollies with many essential nutrients for their health and growth. They also benefit from the buffering capacity of these minerals, which helps to keep the pH level stable in the tank. Soft water should be avoided when keeping mollies as it can cause stress and even death due to its lack of important elements required by mollies to thrive.