Water pumps are the heart of a recirculating hydroponic system, they allow water to flow through the system and deliver nutrients to your plants.
Air stones and diffusers are used to oxygenate the water or nutrient solution in your grow system and/or reservoir with the help of an air pump. The bubbles and water movement they provide helps to circulate nutrients in the water and increase the growth potential of your plants, it will also prevent algae to form in your systems and maintain water temperature.
Nutrient solutions stay healthier and last longer when they are oxygenated.