I thought about adding an air pump, but decided to replace the unit instead. Before that, one mod I did that improved performance was remove the molded 90 input tube and clamp 3/4 in tubing straight in from the pump. This moves more water and makes more bubbles. Keeping the water level low (half way between the taper line and the red collar below the collection cup) gave me more foam and nasty dark skimmate. Following the instructions and setting the water level inside the collection cup cone is a recipe for disaster.
One big problem these units have is that all the tubes are too small. The venturi fitting inside the pump attachment is too small and clogs easily. This reduces the amount of air, raising the water level and... well you know, flood! The drain tube clogs easily, filling the collection cup, and the vent holes in the collection cup are too small and easily clog with foam. Arrrrg!