I myself have never used co2, I just try and make sure my ventilation is adequate it will pull in enough fresh air.(I have also not used a room as large as yours). I am not the most experienced grower, but I would say the one that is wilting is due to lack of water or too much water. It looks like your growing hydro...Are you using flood and drain?(check to make sure hoses are not clogged) If the roots are continually sitting in water you should have an air pump going. These are pretty simple things, and I am pretty sure you would have them covered if it were the problem...Myself I have never had a plant wilt on me, so never tried to figure out the causes...even if using flood and drain you should have an air stone in your reservoir.
Sorry I can't be of more help...
Personally I would chop the taller plant as to distribute the light better to the rest of them... Others may have better ideas for you.