I am curious as to why a waterfall type operates on cascading water but submerged upflow types operate with bubbles. Why doesn't an upflow scrubber run well using a pump to push the requisite volume of water up and accross the screen. I can control the water flow velocity and the "impact" of the water accross the screen by choosing pump capacity and the design of the screen holder. My current upflow is a 1" deep acrylic box (6"w x 10"h) that resides in my sump lit by LEDs through the sump glass. Airstone are a PITA and I want to remove the air pump and drive it with a Rio pump feeding into a plenum that distributes the flow accross the bottom similar to a waterfall type distribution pipe.