I use two 2 liter bottles, with 2 cups of ordinary granulated sugar, 1/2 tsp of yeast and 1/2 tsp of baking soda. My goal is to have each bottle produce well for 2 weeks, so I can change one bottle per week, same day of the week, always at night after the lights go off. I get 1 bubble every 2 seconds from each bottle, or 1 bubble per second from the two of them. I used to use half the yeast, and no baking soda, but got much less CO2 and not nearly a steady production. With your 20 gallon tank, I would use 1 liter bottles, using one cup sugar, 1/4 tsp yeast, 1/4 tsp baking soda, and would change one bottle a week.
In my opinion DIY CO2 is not steady enough in production to ever use with high light, so we don't need really high concentrations of CO2. My tank is a 65 gallon tank, and my plants grow many times faster and healthier with my CO2 system. I wouldn't use that size system on a 20 gallon tank because I think it might end up giving too much CO2 at night, with the lights off, unnecessarily bothering the fish.