This is some info I've come across on the Internet, it's no big deal but everyone is considering CO2 additions to their grow so I thought it might be useful. Below is the max amount of CO2 plants can break down under given light conditions. Note that it is not the lumen value of your lamps but rather the value the plant receives. And while the upper leaves are @ 5000, bottom leaves might be @ 1000. 2000ppm @ 5000 lm/f2 1500ppm @ 3000 lm/f2 300ppm @ 1000 lm/f2 5000lm/f2 is the illumination value of the sun. The illumination power of lamps decreases over distance. I've read that this value halves with each feet. This is no issue for the sun since a couple of meters doesn't matter after ~150mil. km. Below is another chart I've found. I wouldn't take this one too seriously. It's very rough estimates. It really depends of the plant build. A tall and lean plant won't be able to receive as much light as a short and fat one. [FONT="]HID Lamps 4ft (120 cm) * Ambient 3 ft (90 cm) 400 PPM 2ft (60 cm) 1,000 PPM 1 ft (30 cm) 2,000 PPM Ambient is 300ppm (400 in crowded cities). Using a little fan is advised to let the leaves come in contact with as much CO2 as possible. !!! NOTE: The above information is taken from a hydroponics website I can't remember right now, it doesn't belong to me, I don't claim any ownership nor hold any responsibilty. [/FONT]