I was under the impression that co2 affected PH, but not KH. With the inherit hard KH of my water, and the fish being more sensitive to KH than PH specifically, I didn't think I had much to worry about. I had read an article on the BAR report that had outlined that the PH drop with co2 didn't have much affect on the fish, as the KH stayed steady.
I haven't seen much stress or low activity from my fish, though I have noticed when I do dose Nitrates on the EI day, the punks aren't happy, so I quickly cut that out. The fish seem to be getting along decently, I am looking to build a 3rd tank to move the punks and 2 of the small featherfins out.
I have been having fun playing with the co2, and trying to get my PPM where I want it. I start at about 7.8-8.0 PH, and have been using the co2 to get it near 7.0 or so.
I must admit, noob mistake, I need a KH test kit.
CO2 does affect your kH which is a an indicator of how well your water will resist changes to pH with introductions of carbonic acid (CO2). Hardness is most key when attempting to breed as it affects egg development from specie to specie but as a general rule for well-being, you want to try to maintain fish in their preferred pH range and deter any fluctuations in pH greater than .3 in a 24 hr period. While fish may appear to be "okay" in ranges outside of their preferred levels, it will eventually do them in. Remember these guys can live close to 10 years. If we're only getting a couple out of them, then something we've done has shortened their lives.
Keep up with the research. You'll get it dialed in.