Just thought to add that I tried sealing the top from air with a cap, and I was getting much finer, stable, consistent foam being created at a faster rate than before, day and night.
It might be just my setup, but here's my hypothesis: the change in diameter of tubing from the 1.5 inch PVC to 6 inch creates the venturi effect which creates a pressure drop in a closed system. This boils the water, forming extra small bubbles from dissolved gasses, which are more stable. I believe finer foam is better, as we want to avoid coalescence and Ostwald ripening into coarser unstable foam.
This was a couple hours after making this change (immediately before, it was not generating any foam).