Sorry we forgot about this man... it isn't like we saw it an on purpose ignored it. Again, sorry man... trust me, I know how much it sucks to keep the updates coming, and having no feedback.

Regarding CO2
You can use it both in vegetative (faster growth) and flowering (you can guess). It's good for both times.

If you're going to use it, you should get a controller for it. BGHydro sells various good ones.

You have to use CO2 during daylight... and CO2 + fluorescents = waste
CO2 *has* to be used with HIDs, and slightly higher temperatures.

Ideal leavels range around 1500 - 1800 ppm.

Hope this helps man... sorry for the abandonment...

Keep it up dude, please... don't be discouraged!

The HPS looks sweet. Nice buds coming your way.