You only need CO2 enrichment if you have sufficient lighting. Surplus CO2 in your grow environment facilitates accelerated photosynthesis only if above-normal amounts of light are available. Long story short it lets your plants use more light, but its only worth it if you have that light in the first place. Run a search for CO2 Enrichment, and you'll find a graph depicting growth with supplemented CO2 levels.
Cheers, Sarge