<P>So you have a 12x8x8 building insulated with a/c and a big carbon filter with large intake/exhaust fan. Thing is with 39 vegging plants how much fresh air do you really and I mean really need??? I have not been running the filter/intake so not much fresh air lately but the plants do very very well and I mean rapid growth. So what is the deal since like I said no real outside air??? Am I missing something. They are in hydro tub with two air stones and under 8x4' HO flours. at 500 ppm. Thanks for any knowledge...</P>