that hydrofarm fan is good but if you have a CF its a little tricky, besides with 2 600wtt lights u must be in the 100's. And besides in reality. from 85-75 your not going to get an increase in yeild or at 70, the only time your gonna loose yeild is above 90, your plants arent going to give you less becuase of 5 degrees. Besides think about where plants grow during the summer, its about 100 degress out all summer here. i dunno but in the natural habitat it grows at about 80. from 85-75 i would say ur not going to see a difference. its all about co2 and nutes and fresh air and good humidity.