I created a powerhead with a home-modified impeller to be a needle wheel. In the end, I decided to feed my CO2 into the intake of my filter instead of using the powerhead, so I thought, "Hey, why not put the powerhead to good use and feed just plain old air into it -- that will increase the flow in my tank, and add a little aeration?" So, now I'm running the powerhead and getting a bunch of tiny little air bubbles floating around my tank. Does anybody see any problem with this? My drop checker seems to be staying green -- loss of CO2 ppm was my greatest concern. Will the air bubbles have any major benefits or possibly cause problems in some other way?

If there are no benefits, or if there are possible problems, I may just feed the CO2 into my modified powerhead, which will have the added benefit of increasing flow in my tank (my filter is a little underpowered compared to what many users do on this site).

Any of you scientists out there know what CO2 ppm you end up with if you "needle wheel" plain old air into the tank water with no incoming CO2 from a canister? Wouldn't it be nice if there were some device that could get us 30 ppm by using our atmosphere... OK, I'm dreaming.

Obviously you can get some CO2 by injecting air, but it isn't going to be anywhere near 30 ppm. The bubbles will tend to pick up CO2 from the water if the concentration in the water is more than a few ppm. That would stop the buildup of CO2. Remember CO2 goes into and out of solution with water very easily, which is one reason we can get so much into the water with an external reactor.