What exactly are they doing differently than before? From looking at it, it just looks like they are acting how my harlequins used to act (jumpy when I got near the tank).
If it was a CO2 issue, I would expect the fish to be near the top and gasping.
I will try that. They are used to me being near the tank, my computer chair is right next to them. This behavior is just unusual compared to their normal milling about. If this is normal behavior as well, I am less concerned.
They behave like this this sometimes regardless of my proximity as they are not timid of me at all anymore since the addition of the gourami, except at lights on and lights out. They will try to school with the gourami and generally think they are billy badasses because he is in the tank now. The behavior change was interesting.
Frankly, I just don't see anything drastically wrong with them. They are moving about with bright/full colors, and they are not hanging around the surface with labored breathing. I would be patient for a few days while continuing with your routine mentioned above on the Co2. Your doing a fine job of watching them, and I suspect that if you haven't changed anything else internally or externally...
A few of them have large belly's, have they bred for you at anytime in the past? or did they just eat? Those with the larger belly's could be females just getting into breeding if the Co2 has dropped your Ph. Sometimes it only takes something like that to get fish to breed. Watch the belly's to see if they go down all at once if it is not that they just fed.