I was interested in getting some CO2 into my grow room since the ventilation is less than ideal. A tank seemed too expensive and prohibitive for a novice grow, and drip systems are complicated to maintain and not steady in release, so I finally settled upon doing a yeast system. This is not anything I came up with myself, but I haven't seen it on GC before, so I thought I'd post my implementation/take on it here. Ingredients: Liter Bottle Dry Unactivated Yeast Sugar Warm Water Air Tubing Sticky Tack First fill the bottle with a mixture of warm sugar water. Next, add the yeast and shake the bottle up. Finally, drill a hole in the cap, insert the air tubing, and use the sticky tack to seal the hole between the cap and the air tube. Every three days, dump half the water, add the same amount of warm water, as well as five tablespoons more of sugar. This will continually produce CO2, which you can see bubbling out. Advantages are that this is cheap, quick, easy, and less fussy than other methods. Disadvantages are that CO2 amount is still limited and for larger grows, this is less practical. Especially since I can't vent my room, this has prooved pretty helpful in the grow and I've been glad I implemented it. Remember to put the end of the tube above the plants since CO2 is heavier than air and floats down.