... I saw one of my two remaining rainbows gasping at the surface. Strangely, my hatchets, ram and last rainbow have yet to show any signs of stress except that all of the fish have become less active and seem somewhat sluggish. Is it possible that the CO2 level is too high and it's affecting the fish in different ways?
Rainbowfish (well, depending on what type exactly) have a higher O2 demand. many fish have higher O2 demands, especially fish from cooler faster flowing rivers. I can't say for sure how sensative to CO2 they are though. Alot of times lack of O2 is the culprit not excessive CO2. Are your plants pearling? Do you have surface agitation (at least at night)? I had a lot of O2 related deaths in my tank that I thought were CO2 related at the time. I now run twice the concentration of CO2 and my fish are happier than ever.