Plants use and need CO2 anytime the lights are on.
My outdoor CO2 levels are about 425ppm.
My Indoor levels are about 750ppm.
Optimal levels for a plant is 1500ppm.
If you are using a tank (say a 20 or 50lb'er) you will need a regulator and a flow meter. There is a calc in the indoor forum I think up in the stickies.
I currently only have the CO2 running in my veg room (bloom room is on other side of house, away from the 'brain') I just give a 2 second shot 3 times an hour. I haven't checked levels (just got my meter) Hence my small shots, not to drown the plant, but gives more then ambient.