yeast CO2 injection

Last week I tried to build a yeast CO2 injection system for my 20 gallons
full of plants tank.
I mixed together one liter of water, 25 gr of beer yeast and
a cup of sugar. It worked (one bubble per second) for one day only.
Maybe I used a wrong quantity of components.
Does anybody know the exact amount of the components?
And what about the yeast? What kind of yeast should I use?
Thanks a lot.