One of the questions that has been repeated raised is regarding what is probably considered the central concept behind the so-called "Upflow" algae scrubber: bubbles. Repeatedly touted as the mechanism that makes the UAS possible. Well here's my current scenario. I have an L2 UAS box on a 40 breeder with 4 clowns and a few corals. Fed decently every other day. This is the UAS Test unit that I originally built and put into service 1 month before the "public release" of the UAS concept (there was a small group of us given a "sneak peek"). It sits on top of the tank, and has water pumped into it. I have toyed with many version of bubble distributors, etc. For a while, I was running a piece of hard tubing with holes poked in it, held down with suction cups. This worked well at first but the holes quickly clogged, so there was maintenance involved. I got lazy and just let it stay mostly clogged. Algae still grew, in fact, better than ever. Recently I removed the suction cups and used a piece of acrylic with holes in it, strapped the line to the bottom of it, and tied the screen to the acrylic above it. I let this grow for 2-3 weeks. Growth was very good. The acrylic with the bubbler on the bottom and screen on the top, growing algae, floated up off the bottom. Algae grew underneath the bubble bar, on the acrylic box itself. Only light and water flow there. No bubbles. I got some video of it during cleaning, and I'll try to upload it as soon as possible. Water feed is via a Cobalt MJ900. Air pump is a mid-size regular old pump. The harvest was about 100g, squeezed. Pretty good. Conversely, I also have a 60g stock tank with 2 clowns, lots of corals, lightly fed, with a SURF2 that I picked up from a local guy who couldn't get anything to grow. I have been cleaning that unit at the same time as the UAS L2. The box is now home to at least 3 bristleworms, each at least 6" long. So I don't know how much that is throwing off this particular test, but it sure makes it fun to clean the thing. Air pump is the Fusion huge sucker that SM sells. Cranked to the max - lots and lot of air. This growth period's harvest: 4g. Four. Not 40. FOUR. I bet if I really scraped and scrounged, I could have gotten at least 6, maybe 8. But that would have probably included a few strings. I consider myself a child of science, so I can't "close the book" on this bubble thing, but if it walks like a duck and quacks like a duck, it's a duck. Right now, looking a LOT like a duck. Duck = gimmick.