I have built a reactor, using an internal filter/powerhead unit. I came with a sponge filter and a canister that you can fill with media. I left the sponge out and left the canister empty. Drilled a hole just big enough for the air line. The problem I am have is the "seven up" effect is bad enough that it bothers me and it is really loud. Why? Is is because the chamber is only about 4" tall and that is not enough room for the co2 to dissolve properly, it makes noise. That is my guess. Kids broke camera...but if you go to ebay and look up
Aquael AquaThrust Powerhead & Internal Filter
you will see the canister I am using.