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.