co2 is used because unlike animals, plants breath co2 and exhale o2. we breath o2 and exhale co2. the more co2 you have in the room the more the plants can breathe, thus more photosynthesis, thus bigger plants. if you use co2 use it all the way through your grow. plants can grow double if not more vs not using co2. thats why it is very important to vent your room and have oscilating fans. if not the plants will use all the co2 in the air and not have anymore to use. i am about to set up co2 in my area.