(I think I understand this properly, my friend a Chemistry professor has quite patiently explained this to me on many occasions.)

Adding CO2 will react with bicarbonates in the water, forming carbonic acid, which does in fact lower pH. I do not think we add enough CO2 to do any danger to plants and fish in the tank, if done properly. (That is, adding enough to be non-limiting, not dumping in a tremendous amount.) Also, how one goes about getting the CO2 to dissolve into the water as well.

My city's water sounds opposite of yours. It is virtually like RO water, with a neutral pH. I add "GH Booster"/Equilibrium and use CO2. By using a drop checker with 4dKH reference my pH doesn't really go below 6.5-6.7 (I have not done any scientific measurements, or at least not consistently).

Fish and plants seem pretty happy, no deaths.

Now, last year when I went away for a long weekend, I had somehow opened up the needle valve on the CO2 effectively upping the dose 4-5x! When I came back the plants seemed ok, but there were rotting fish carcasses all over the tank... *Except* for the Goo-Obo gudgeons! Tough little mo-fo's.

-Jason