CO2 & Air Pumps
Air pumps pump air into a tank to disrupt the water tension, to release bad gases and to put oxygen into the water. CO2 injection provides additional bio available carbon in the water and CO2 is usually the limiting growth factor in aquarium plants.
CO2 is not 100% needed on a planted aquarium. It depends on what kind of plants you are keeping, how big the tank is and what kind of lights you are using.
If you have an aquarium filter that runs on an air pump, an air stone can be beneficial. An air stone will create an abundant cascade of small to tiny bubbles rather than a limited number of larger bubbles. These smaller bubbles will be much more effective at moving water through the filter to make the filter more effective and efficient.