I built the venturi diffuser last night. I am not sure its doing what it should. Should there be a space at the top filled with CO2? If not, then I don't see what the tube from the side to the input of the powerhead is for??

What I have is maybe 1/8" of CO2 at the top of the big tube (which was from a big-al's vaccum thingy).

What happens is bubles come in near the bottom, get swirled around A LOT until the dissappear. Nothing much seems to happen re that extra tube???