I don't see how skimming affects the rate at which CO2 leaves the water, as long as the skimming doesn't ripple the surface excessively. My constant water change system skims my surface constantly, and I have no trouble getting to 30-40 ppm of CO2 in the water. I do think I run a higher than usual bubble rate doing so though. If I had DIY CO2 that might be a problem.