extractour fans are a big job i agree but all you have to think about is circulation use ure wits, draw a diagram of air flow in ure cabinet,have fresh air blowing in from underneath the plants, so they can make use of all the co2 and then stail air blowing out abouve your plants leaving the grow room one way or anouther. you dont need extracour fans for good air flow.also home made co2 very good idea bvut again with the right venting your plants should manage to grow suffiiantly,but saying that the more co2 the better.