I've mentioned this before, but I use an upside down terra cotta pot slightly tilted to great effect. I run a tube right from the CO2
bottle (mine is a DIY 2L). I think the trick is in the pores of the terra cotta as it does not just fill up and then burp little bubbles like some bell filters do. The pot will build up with a hundred or so bubbles, and then burp almost all of the bubbles in a giant bubble up to the surface, and then refill. This allows for a LOT of contact time with the water and I routinuely have over 20ppm CO2
in the tank. I'll see if I can find a pic that shows the setup....
Look in the back left of the tank to see the pot...
After getting a couple more plants and 2 weeks of CO2
injection (makes a HUGE difference)