I'd better check that answer....
ok, from this site
http://www.thekrib.com/Plants/CO2/ (with pics!):
Reactors
Once you have chosen your CO2 source, you must get it to dissolve into the aquarium water.
The easiest, if not the most esthetically pleasing, method is to trap the gas in an upturned cup or ``bell''. While trapped, the gas eventually diffuses into the water
Another simple approach is to run the CO2 line into the intake of a powerhead of canister filter. The impeller helps break up and dissolve the bubbles.
You can build a gas reactor, which forces the CO2 to quickly mix with rapidly moving water in a small chamber. I (EO) wrote an illustrated article with construction details. So have other people.
Relatively new on the scene are ceramic/glass diffusers, available from ADA or Eheim. This method replaces both the reactor and metering valve entirely; The diffuser passes pressurized gas as tiny bubbles through the glass. There have been debates over a possible end-of-tank dump effect if you are not also using a good needle valve to regulate flow.
Finally, here are some more diffuser and reactor postings for more info. Nadeem Faiz drew up a nice illustrated diagram showing a full yeast generator, bubble counter, and bell diffuser. Incidentally, you might want to look at what kind of hose and valves you use to transport the CO2 to the tank. Cheap rubber check valves are known to disintegrate quickly from the carbonic acid!