Over the last few months my school of 7 cardinals has become very shy, hiding behind plants in a corner most of the time. When they're out, they dash for cover at any sign of movement outside the tank. The pygmy cories are a little less shy but still similar. The chili rasboras are completely fearless though.
Back in the winter I was running two T5HOs, a Koralia Nano, and the filter full out. Now I'm using one bulb, no power head, and the filter at a reduced rate, so I would have expected the shy fish to be MORE active, not less.
CO2 is not an issue since I have a thriving breeding colony of SR shrimp, plus a couple of amanos which normally seem to be extremely sensitive to CO2. They're all fat and happy.