Ok let's take this thread further into the light.
Just putting the CO2-tubing in the pump without an airstone accomplishes 3 things:
1. The obvious - supplies CO2
2. The bubble counting.
If you listen carefully you can hear each bubble hitting the impeller. The sound is very gentle. You get used to it and tune "in" to it only when you want. I'm sitting here about 10 ft from my tank and I can hear the sound if I want. But don't think that the sound is loud. Even if you stand right in front of the tank you can't hear it unless you mean to. The sound is like a pinch of sand being gently dropped on paper.
If you diffuse the CO2 at the end of the tubing the "bubble counting feature" of this DIY diffuser of cosmic proportions will disappear. You will not get any better efficiency out of it with an air stone. The magic is in the sponge being over the pump outflow. The CO2 gets trapped there and only the tiniest bubbles escape (if any).
3. The power failure protection
Yes, this legendary design has a built-in power failure protection too. It will not allow the CO2 to be difused if the pump is not working. If the pump is not working there could be a power failure and there is no water movement. If you continue to disolve the CO2 as usual say good bye to your fish.
Without an air stone if the pump stops working the CO2 bubbles come out of the end of the tubing and do not get chopped up by the impeller. They find their way out of the pump and float to the surface. No solubilization takes place. Your fish are safe.
By the way one of these pumps is enough for a 75 gal. tank. That's with pressurized CO2. Old Skool DIY CO2 brewers should not feel a need for a second pump. But hey! It's so cool!
75g. tank with only 1 (one) "tiny-super-efficient-imperial-panda-supper buffet-looka lahka man" reactor: http://picasaweb.google.com/ddasega/DaveS#
I'm now working on this same design but made of pure gold. Exquisite item for your aquascape. Only for the true connosieur.