Airpumps on DIY CO2 Setups

Some time back, someone asked about using an airpump to pressurize a DIY 
CO2 setup.  Another writer suggested that the potential to pump foam into the 
tank is a good reason to avoid doing this.

A better reason to avoid it may be that it will make the CO2 system almost 
totally ineffective.  This is because mixing the CO2 with normal air will reduce 
the concentration of the CO2 pumped into the tank. 

The purpose of the yeast/sugar mixture in a DIY CO2 system is not just to 
produce CO2.  CO2 is already abundant in the air around the aquarium.  This 
CO2 in the room air dissolves into the water in the tank and the concentration 
of the CO2 in the water reaches an equilibrium with the concentration of the 
CO2 in the air.

We want more CO2 in the water than that.  To get additional CO2 into the 
water, you must produce a gas with a much higher concentration of CO2 than 
that found in room air.  This will alter the concentration equilibrium set up by 
the room air and result in more CO2 dissolved in the water.  The sugar/yeast 
or CO2 cyclinder provides this highly concentrated CO2.

Given that most DIY CO2 setups produce a bubble of CO2 every second or 
to, using an airpump that produces many, many bubbles per second will 
reduce the concentration of CO2 until it is about the same as that of the air in 
the room.  You may as well dispense with the CO2 setup completely.

I'm surprised that the chemists etc on this list, who know more about the 
subject than I, have not pointed this out.  Is there something wrong with my 
thinking on this?  Check me out, please.

Paul Chapman
Saskatoon, SK