Fluctuating CO2 levels from day to day during the photo period don't harm the fish, but they do cause algae blooms. That is why DIY is so hard to use without using several bottles with staggered start times.

Yeast is a living organism, and it does the consuming of sugar to make alcohol and CO2. Bacteria is a different type of organism. (I'm not an expert at all on that subject, but I think I am correct.)