Quote Originally Posted by dude0
Does fresh air contain co2?
Yes; but not above the 200-300 ppm that is in the air by nature. To grow successfully with co2 you need to get it up to 1500 ppm and keep it there for the light period. Then run exhaust and exchange the air and do it all over again the next light period. Dry Ice does work and is easy to use, it can add up though. Like 5 pounds a day for the whole period get expensive. Even if you only used it every other day to save money you would see a big difference in the end.

Peace