I use the yeast/sugar method. I have made home wine in the past and have the kit already so it was the cheap option for me. Yeast also likes a slightly acid medium to grow in so my recipe is....

1kg white sugar
4.5 litres water
1 tsp citric acid (juice of 1 lemon would do)
yeast nutrient
dried breadmaking yeast - 1 teaspoon.

boil the sugar in a couple of pints of water to dissolve and add the nutrient and acid. Mix up to 4.5 litres with cold water and when temperature is warm (not hot) add yeast. Put the cork in and position the other end of the hose above your plant. At the temp of my grow space I expect this mix to keep fermenting for a couple or three weeks....trickling carbon dioxide right where she needs it 24/7.
At the end, when the sugar's all used up and the yeast has settled out of the water you will have a solution of alcohol (about 10%). It'll taste of little except yeast, but only local laws stop you distilling. Could make your own tincture if you were inclined I suppose!
sawmiller Reviewed by sawmiller on . CO2 solutions for micro grows I'm growing in about a 1.4x1.4 foot area where the plant pretty much takes up the entire area. I've heard how much CO2 can help a grow throughout all stages of development and thought it might be worth it to add some CO2 capabilities to my next grow. Do any of you have ideas about CO2 sources that are available short of just buying a cannister? Thanks. Rating: 5