I actually made a couple small changes (sorry, couldn't leave well enough alone). I cut the photoperiod from 12 hours to 10 hours. A 26 watt CFL only 8 inches from the HC might be melting it a little, and I removed the Co2 injection. I was concerned that maybe the Co2 was making a hypoxic (oxygen free) environment and I think plants need to respirate oxygen at night.
I poked some holes in the top of the plastic wrap to facilitate gas exchange. I think air probably has plenty of Co2 already so why mess with mother nature?