I use a non-traditional method of co2 injection with my setup. It is a constant "drip" injection that is set on a timer to be on when lights are on. Since I have constant intake/extake airflow which is neccesary to keep the heat down, I have to constantly emit co2 to be effective. I use about 20-30 lbs of co2 per grow but for me it's free, I get it at work.

If you have the kind of setup where you can shut off the intake/extake for a little while, without having any heat issues, then use the traditional method of using co2 (with hoses hung above your plants). You will conserve co2 this way. I have to constantly "drip" co2 to my plants or they will never get it.