Heres the best home made co2 recipe I've come across:
* 10lb. white sugar
* 5 gallon clean bucket W/lid
* 4 1/2 gallons of water
* A piece of toast browned and hard
* A table spoon of dry active yeast (for baking bread)
First boil the water, (this will ensure clean water) remove from the heat and add the sugar. Stir until sugar dissolves. Let cool until water is room temperature (if you don't let it cool down it won't work).
After the sugar water has cooled, float the piece of toast on top of the water. Now, empty the tablespoon of yeast over the toast. After a few days, the yeast will take over the toast and start making bubbles (CO2) in the bucket. After a week, the amount of bubbling (CO2) will increase.
Keep the lid airtight on the bucket. CO2 travels up the dispersion tubing, and due to it being heavier than air, falls directly onto your plants. Timing your exhaust, is essential in maintaining an effective level of CO2.