(I read 'in-tank diffuser' - hope you don't mind if I offer an alternate opinion even though it isn't atomic )
If you're looking for a small footprint my approach may not work for you, but it does save on the cleaning soak needed for those diffusers (I think it is every 3 months). I didn't want this hassle, so I bought a Rio 800+ PT (these things are fairly cheap at $20 or so). This is a needle-wheel powerhead which takes the line from your CO2, directs it to the impeller of the power head, and uses a venturi line to suck the chopped up gas bubbles out of the impeller chamber and expels it through output with the flow. This approach has the added bonus of adding much needed flow to my aquarium. The other caveat is that this will generate microbubbles, producing that soda-water effect -- but if you are looking at those atomic diffusers you are likely expecting that anyway.