An air pump produces tiny bubbles that rise to the water surface and cause surface agitation. Surface agitation helps create a healthier environment and provides breathable oxygen for your mollies. Additionally, these pump-generated bubbles help with water filtration, so the tank will stay cleaner for longer.
Additionally, you should add live plants to your tank. Aside from the plants promoting oxygen exchange in your tank, mollies also love hiding and swimming around live aquatic plants. The addition of plants will make your mollies healthier and happier.