Did you use any check valves in your set up?
I've not tried diffusing with an HOB filter, but there's a lot of surface agitation associated with that style of filter, so i'd think you will be out-gassing a lot of the co2. Using a wooden airstone at the end of your airline tubing would work much better. Or, i've read many times of people using a piece of chopstick shoved into the end of the airline tubing.
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Pimp #418: 2075, 2217
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