Mmm... the bit about adding Oxygen causing CO2 concentration to fall, well, I think it's a big question for a lot of AHs, but I don't think it decreases the amount of dissolved CO2. I think Henry's Law says something about this, but I can't remember the exact idea and application of the equation. I've been trying to understand it.
Seems like if you add Oxygen, you just increase the total pressure and the partial pressure of O2 in the water. Like i said, this is a big question but I don't think the added O2 displaces any CO2. Thanks for your patience.
Oh, by the way, I can't reallly make an educated guess at how many bacteria are in a 10 gallon tank, but there are a lot. Since bacteria are very small, there may be room on surfaces for hundreds of thousands of them, probably more like millions.