You need more light and running a closed room will take CO2.
They will grow with what you have and without CO2 but the CO2 will pay for itself many times around in your setup and MUCH more light will as well. Electricity counted for.

I would not run that room with anything below 1200W HPS (3*400W).
3*600 would be better and it will pay for itself easy.
All that if you want to really use some of the potential you can get with such a nice grow area you have there.
CO2 is a bit of a hazzle but think about the fact that it WILL increase your yeild with 50-150% compared to a normally vented growroom and in your case the gain might even be bigger since if you don't you will have an oxygene tank in there with very little CO2 in the air.

Plants are made of mainly carbon and 100% of that comes from the air. No CO2 no growth.

Puffzter