Darlin', it's CO2. Plants breathe Carbon Dioxide, like we breathe oxygen. How effective it will be depends on the size of your grow area, the temperature of the water and how air tight the grow area is. In my little clone/seedling cabinet (about 6 cubic feet), I use a milk jug with yeast in it. I add a spoonful of sugar a couple times a day. It fizzes right up releasing that good CO2! In a small space, it will work fine.- Granny:hippy: