The ole CO2 convo.
I have used CO2 very succesfully. No, it wasn't candles, aquafiz or dryice.
In the "outside" most CO2 levels are around 300-500 ppm. This is normal out side air. Obviously, this is more then efficiant for plants, as plants do great outside!
So, if you have a growroom, and exchange your air in your room properly, then the CO2 levels in your room should match the CO2 levels outside.
This is sufficiant to grow a healthy crop, and MANY folks have acheived great results and yeild doing nothing but this.
Now, if you wish to have a boosted CO2 level in your growroom, then, to do it PROPERLY, there are a few things you MUST do and buy in order to make it worth while.
Your grow room MUST be sealed. This means that no air leaks in the room, and your light ventilation. If you air cool your lights (which usually MUST be done running CO2), then it must be a closed loop system, meaning you pull air from out side the grow area, thru your lights, then exhaust it out side the grow area. There is the light requirements.
Next, you must have a good exhaust blower that is controlled by a humidistat.
That way, the exhaust will only run when the humidity goes above the setpoint you set.
For your intake (usually passive) you should have a "flapper" type intake hole that auto closes when air is not being drawn in. I forget the true name of these....
Now, you need all the CO2 equipment. Burners or Bottles. The only 2 real choices. You will also need a CO2 controller. I perfer C.A.P. controllers. You can buy one that controls all aspects of the room ventilation, along with CO2 dispersment, all in one. Look for one with "fuzy logic" for best results.
Another fun fact is to properly use your boosted CO2 levels, your grow room temps must be between 85-90 degrees, a bit higher then u would normally run without it.
There are a few other details, but that is the jist of it. Anything less then that, you are wasting your time and $$.