In order of preference.. (highest to lowest)
1.) Other (read below for more info)
2.) CO2 into canister filter
3.) CO2 into HOB filter
4.) Other (read below again)
5.) Airstone
6.) Bell
1.) I essentially do what Gulf described. I use an Aquaclear 802 powerhead (its what I had laying around) with a gravel tube attached to it at a 90 degree angel with a sponge stuffed in it. Works great, and it was cheap. Its good for larger tanks.
2.) This method isn't too bad neither, it minimizes CO2 loss since a canister filter is sealed (or it should be), the impellar within the filter chops up the bubbles nicely, and the filter media provides good spots for resistances.. increasing the amount of time of contact time the CO2 bubbles have with water. Which essentially gives you nearly 100% diffusion. Ranked #2 since some people don't have canister filters, it can be somewhat noisey (with the impellar making noises from the bubbles), and some filters might experience air locking. Works for larger tanks.
3.) Works ok. From what I hear and read, aquaclear filters are best used for this method. Essentiall the same as a canister, except the system is not sealed, so there will be CO2 loss during the diffusion process. Not only that, but the filter might cause some excess surface aggitation, leading to more CO2 loss. Attach a sponge to the outlet of the filter to reduce surface aggitation... I do that on the AC mini I have on a tank in my room... there's almost no surface aggitation from that filter right now. I would suggest using this method for smaller to medium sized tanks. It can be somewhat noisey, since the impellar is chopping up the CO2 bubbles.
4.) Powerhead diffusion. Works pretty good also... the impellar of the powerhead chops up the bubbles nicely, and shoots them out as a mist... you can also attach the CO2 line to the arriator of the powerhead. Works fine for smaller to medium sized tanks, it can be somewhat noisey with you feed the CO2 via the intake of the powerhead.
5.) Airstones work ok... you need to find airstones that will give you the smallest sized bubbles possible... wooden airstones should do the job. It also helps if you can direct some water flow over the path of the ascending bubbles, to increase the amount of time the bubbles are in contact with the water. I'd suggest this method only for small tanks.
6.) I tried the bell method once, and I'll never try it again. The bell takes up far too much room, and in my experience very inefficient (much more than the airstone). CO2 would collect under the bell.... and continue to collect, until a large amount of collected CO2 escapes from the side in the form of one large bubble. Even with waterflow underneath the bell, I got little diffusion (compared to even an airstone). It might work for you, but I wouldn't suggest it.
HTH
-Richer
