Time to talk about air. Your plants need lots of Carbon to make the sugar that powers growth.

The Carbon comes from the CO2 in the air. Your plants can use a lot more CO2 than what is in the air normally. That's where adding CO2 comes into play.

If you increase your CO2 to 1500 PPM you can increase your plants growth rate. I went from 12oz. to over 16oz. per harvest using CO2.

Don't mess with yeast or tablets or all that other crap. You have two real options.

1) CO2 tank

2) Propane burner

I've never used a propane burner so I'll talk about using a tank and regulator.

The cheapest way to use a tank is with a timer. Go to this site and put in your room's dimensions.

CO2 Calculator - Greentrees Hydroponics

Figure out what it would take to fill your room in 15 minutes and set your valve to whatever the calculator tells you to. Set your timer to come on for 15 minutes 5 times a day. Keep it off during the night.

You can check your CO2 level with a testing syringe. You can buy them at most hydro stores. You draw in 100cc of air and then push it through the glass test tube. You read the purple band and it tells you your CO2 PPM.

By far the best way to control CO2 is with a controller. A good controller checks the CO2 level and turns on the tank regulator when needed.