Yes your correct Venturi on the outlet, otherwise the air on the impeller would cause cavitations and water flow would be greatly reduced just like on a boat jet drive. My earlier comment about the impeller breaking the surface tension with air was an afterthought of how the O2 increase might also be at play besides the bubble surface area interaction. But there are several new papers out now on how air bubble surface area and rupture plays a much bigger role on dissolved O2 in water then previously thought and why water treatment plants rely heavily on underwater forced bubble aerators to provide O2 to aerobic bacteria in braking down waste. Anyway the proof is in the test results and I can tell you it shows in my fish as well especially the large body fish like adult 3” barbs. They are definitely out in the open more and not resting as often and especially after water changes no more heavy breathing for 24-48 hours . I do admit that prior to using a forced powerhead aerator I did use bubble wands, but I suppose the volume or agitation just wasn't enough, the powerhead pouring out a a stream of heavy bubbles at 270 GPH has made all the difference in dissolved O2 readings in both my 60 and 120 tanks, so much so I’m buying a 3rd for my 80 tank and have removed all but 1 air wand. The only down side is the bubble wash and current which I solved by directing the flow against the back glass and behind my HOB lip underwater as per the photo I posted earlier. This has greatly confined current and bubbles wash to one corner of the aquarium leaving the rest of the tank relatively calm and now the fish pick and choose their location, i.e. the Otto's loving to be in the current and the angles away from it.
