Looks like that exhaust ducting is blocking some of the light from getting to the plant on the right. If it is an enclosed area, you'd be fine with just a dayton blower(
http://www.bghydro.com/BGH/itemdesc....AEF140C&eq=&Tp) inside the box blowing out the top.
Going by an eye-d out estimate of your dimensions, the blower I have linked for you will cycle all the air out of that box 3X/minute, not to mention provide a nice breezy atmosphere for your plants.
