Is the swiveling joint of the output tube above the waterline? Mine is, and I think that's the source of the massive infusion of bubbles this filter causes in my tank. The solution stated on their website is to raise the water level of the tank, which I can't do because the level of the tank is determined by the overflow box.
It did take me several tries to get the O-rings to fully seal (the canister was filling up with air), although since then (even with having taken it apart several times to clean the micron filter) it's been working fine, aside from the bubbles.
It doesn't bother me much since I only run the filter when water polishing / suspended silt removal is needed. But I can definitely see how that would annoy someone trying to use it full-time.
I wonder if something can be done to seal that joint... maybe get it set in the right orientation, then put some silicone sealant around it? Or, try to smear some of that silicone grease stuff into the joint?
--Mike