At first I heard that surface agitation will greatly decrease CO2 levels. Then I was told that it doesn't really matter, as long as your setup isn't mistaken for some white-water rafting experiment... So really, I don't know. I think #2 is probably right though.
If you aren't adding CO2 to the water with a DIY or pressurized system, the amount in the water will be less than 5 ppm, and that will come mostly from the atmosphere. In a Diana Walstad tank you might get a higher level, due to CO2 release by breakdown of organics in the substrate, but I doubt it ever getting near 10 ppm.
Water surface agitation has to help keep CO2 in the water as long as the atmosphere is the only source of CO2. But, if you add CO2 some other way, agitation causes a loss of CO2 down to the equilibrium value with atmospheric CO2.