well I have grown for 6 years in soil and I have used a air tight closet for the last 4 of the 6 and I have used co2 for 3 out of the 4 in the closet , I had a problem with heat in my closet but then I found out temps. need be a little hotter for plants to use co2 well....so problem solved...lol..your plants will look more lush and snap back if there is any problems....but with co2 the plant will use a lot more of everything it needs to live... its like us on a human growth hormone ,,,,everything is excelorated. if you have enough light , little hotter temps, and a good feeding plan .... co2 will shove a boot in your ladies asses and make a jungle out of your grow box in no time