Location of the air pump

Air is 75% Nitrogen so you're also giving N to the roots. Do not worry, they won't uptake anything not designed to do. Besides, CO2 on the water is not a bad thing, many growers water with seltzer/carbonated water because that will release the CO2 and be taken by the natural photosyntesis of the plant.

Hey bro! 👋 If I were you, I wouldn't touch anything and wouldn't worry about a thing! 👆 When growing on an industrial scale (from 1 hectare and more), they run a hose for CO2 supply along with the irrigation at the cultivation site. During flowering, carbon dioxide is delivered directly to the roots by burying a pipe in the topsoil. The plant processes carbon dioxide not only through its leaves and flowers but also through its roots! 👆 So there's no need to worry, your roots will take in CO2 if the plant needs it, just like the leaves and flowers of your girl will! 👌 Good luck, bro! ✊😇

C02 does sink but as long as your fans are moving air your fine. Cheers