I'd use one 2 liter bottle. I'd start with no more than 1/8 teaspoon of yeast. My formula for your nano tank would be 1.5 cups sugar, 2 1/2 cups water, 1/8 t yeast, 1/4 t baking soda.
If that doesn't give you enough CO2 then up the yeast (and the sugar, too if you want). There are many formulas mostly based on these basic ingredients. They all basically work so start smaller and gradually increase it to what works for you.
Okay thank you. Most of the formulas I had found online were for 10 gallons and up. I'll definitely try this
Just remember that it is the yeast that is the the producer of the CO2. The sugar is yeast food. Generally less yeast gives less CO2. It is not an exact science and getting the right amount for your tank and requires experimentation. I keep notes of my formula adjustments and the results from week to week.
Also I find that my yeast CO2 output increases the first couple of days.
I forgot to ask if you have fish in the tank or is it plant only? If you have fish a drop checker and careful observation will help you monitor CO2 level and help you manage your fish's well being. My fish are fine, but that was my greatest concern in adding my DIY system to my tank.
I'm still pretty new to this so I welcome any others who may weigh in with more experience.
It's going to be a err cherry shrimp tank. I have two in there right now. They are my concern which is why I want to get this right and not harm them.