I am using Duetto DJ-50 internal filter for diffusing CO2. Its a very small
filter with an option to inject air line. I have a picture of it in my
article

http://www.dlink.org/aqua/CO2.html

--
Thank You

Dominic
http://www.dlink.org/aqua

I just coincidently happen to see one of these things in action while
at the LFS store today. The first thing I noticed was the CO2 bubble
mover system inside the tank. It's very cool to watch....but, the
more I thought about it, the more it seems like this type of diffusion
wouldn't be any better than a 'bell' type diffuser....maybe worse?
The bubble just reaches the top and goes to the surface

The canister was nice tho. Maybe too small for a larger tank but
perfect for something like a 5-10..maybe 20 gallon.

My little 1L reactor is pllumbed into the bottom of the venturi of my
power filter and is giving me 20-28ppm. I can't imagine doing much
better....actually, any higher would be a concern