I'm currently using the system described below (converted Elite Mini filter) to diffuse CO2 into my tank.
http://www.aquaticplantcentral.com/forumapc/do-yourself-aquarium-projects/44053-tiny-super-efficient-co2-reactor.htmlThe basic idea is like Jub's thing. I have the CO2 airline going into the bottom of the filter, attached to an airstone, so the airstone breaks the CO2 into small bubbles, that are smashed up by the impeller into tiny bubbles, that shout out the filter exit and (mostly) get caught in the sponge over the outlet.
As I understand it, the idea is that if you use a sponge to catch the bubble, rather than have them shoot around the tank, more of the CO2 in the bubbles gets dissolved by the water exiting the filter, so you get better diffusion. It makes sense to me. My diffuser gets a few tiny bubbles floating off the top of the sponge with each CO2 bubble going into the system, but I know that most of the CO2 stays in the sponge until it dissolves, because when I squeeze the sponge all this CO2 comes out.
But if you read the thread (its quite long), some people seemed to think it's better to not use the sponge and have the bubbles shoot around the tank. I've not tried that (yet).
Does anyone have an opinion of the benefits of having little bubbles shooting around the tank (and then presumably rising to the top) as compared to having little bubbles caught in a sponge which has water flowing through it? I've have thought you'd get more efficient dissolving of the CO2 with the sponge and better spread of it around the tank without the sponge, but not sure.
It's interesting!