Im using dry ice currently as we speak i just changed it from the cooler to a bag with a couple holes and its hanging from my top its right in between the plants,i figured they would get more c02 if it was elevated since the c02 seems to stay low when its evaportating, i bought 7lbs for 1.39 at albertsons and its still going from early this morning...im didnt know why they wouldnt want c02 at night??