I've heard that if you wanna run co2 you need to be in a sealed room, or at least shut down your fans for a while while you release the co2, and the fermintation method sorta just releases a slow steady stream...If you're intake and exhaust change the air out once or twice a minute, or even up to once every 5 minutes (I'm sure your setup falls into one of those ranges) then you're exhausting the co2 before the plants get a chance to use it. I once saw someone who rigged up a 5 gal bucket with 2 computer fans, one in and one out...He let the co2 build up inside the bucket, and then he had a timer turn the exhaust fans off, and another timer turned the computer fans on in the bucket to disperse the co2 while the exhaust fan was off, for like an hour or so I think...I dunno, I was all for the fermintation co2 method until I did assloads of research, and I came to the conclusion that no sealed room, no co2. But shit, half the crap on the internet is bullshit anyways, if not more than half...So maybe it will have beneficial results after all.
