Your not growing in the environment needed for additional CO2, what your giving the plant now is plenty. If your really looking to score big, start your FIM now, thats only if your serious about getting the HPS lights though.

Anyway, I don't use CO2 until after they start the actual veg stage, and your just about there. If you want to start with something simple follow the 3:1 sugar to yeast ratio with 5 parts water for 1 part sugar, you can use a 2 liter, grab a nail and hold it with pliers, heat the nail, poke it through the cap of the 2 liter bottle should be tight, run a 1/4" tube through it, put the other side of the tube at the base of your plant and tie in place with a bread bag tie.

This is Ghetto Co2, the real stuff costs about $325 to get started