CO2 is used during photosynthesis of a plant (energy production). One product that it makes from this is O2. The reason it is good to have more CO2 is because it gives the plant more materials used to grow and make energy. Your plants do have to have it because if they dont they will cease to grow. The easiest way to keep levels high are just keep a fan circulating through your grow room so outside air can get into it and replace the air inside the grow space.