i have the same thing in my tank. the film is composed of protiens from fish waste, leftover food, etc. in a fish tank, there is no mechanism to carry this away, so it collects at the surface. breaking the surface tension will mix this up, allowing it to be filtered out, but you are correct that it also de-gasses CO2. in my tank, i use a aquaclear mini set on the lowest possible flow rate to mix this up, and it seems to have a minimal effect on the CO2 levels in my tank. there are other solutions, such as surface skimmers that attatch to the intake of your filter, but the one that i tried made so much slurping noise, that i went back to the aquaclear. i have, however, heard good things about the Eheim seurface extractor, but i have never used it. some people do nothing about the surface film, but i think it looks gross, and if thick enough, i would think it would negatively affect gas transfer.