Here's a copy and pasted of things you need to know about CO2 before splashing out! 
For CO2 to work, you need....
 
-  Bright Light - Your cannabis plants need a lot of  light for CO2 to be effective at speeding up growth and increasing  yields. As far as light intensity, we're talking about the kind of light  produced by a 1000W HID grow light. CO2 only helps plants make more  energy from bright light, so it won't help if you have low light  levels.
 
-  Enough CO2 - You must be able to maintain 1200-1500 PPM of CO2 in the grow area or it won't help your plants.
 
-  Sealed Grow Space - You must be able to completely seal your grow space so CO2 can't get out.
 
-  Temperature - You should maintain temperatures between 85°F (30°C) and 95°F (35°C) in your grow space for CO2 to be most effective.
 
-  Humidity - You must keep humidity below 60-70% to  prevent mold and other problems, but keeping humidity low can be  difficult with a sealed room while running CO2. You may need to get a  dehumidifier.
 
 I know it may sound like a lot, but these factors are important! If  you have the wrong setup, adding more CO2 isn't going to help you! Today  I will share everything I wish I’d known about CO2 when I started  growing, so you'll know exactly how you can use CO2 for faster growth  and increased marijuana yields. Let's get started!
 
Beware! Some  people (especially companies selling poor CO2 products) will try to  trick you into thinking that CO2 is some magical supplement you add to  your grow room to make plants grow better. Unfortunately this is not the  case! There are good ways to add CO2... and there are ineffective ways.