As Waterdog says, smaller bubbles are more efficient; they move more water for any given airflow. You can or should only turn down the flow so much. Too slow and it's not filtering much at all.
If you've got it adjusted to just ripple the surface, rather than turn down the flow (and diminish the filtration) you're probably better off putting a flat open sponge above the lift tube to catch the bubbles and smooth the flow. Juditko will tell us how that pre-filter sponge works over time. Java moss is as good and I like the way it looks.
If you're curious, you can actually determine the flowrate by holding the filter at an angle and letting it flow into another container. Time it and measure it. ATI Hydro explains how to do this. That will be the minimum flow you can expect at that setting.
If the surface is rippling, that's an adequate flow rate for me.
A sponge filter bio-filters well, with less flow than a typical HOB, because it uses a relatively larger foam block for housing nitrifying bacteria. A good HOB can be a better mechanical filter. Many serious keepers use both.
Last edited by Hallyx; 02-20-2013 at 02:50 AM.