ok, if there was a hole in the return plumbing, where it can suck air in like a venturi. Then you should be able to see where it is easily. Just restrict the outlet in the display, and it should spray water out. Do not block the flow completely as you might pop the plumbing. But I doubt that a hole is the problem.
The pump inlet, does it not suck in bubbles. How far is the water dropping over the last divider. How full is the return chamber? I had a problem when I upgraded my return pump where bubbles pulled down as the water flows over gets into the return pump. It completely micro the bubles and I had these fine bubbles in the display as well. solution was simple, just run the water level in the return chamber higher and repositioned the pump so that the inlet is in the furtherest position away from the overflow.
The nano I set up got a similar issue. The crappy little skimmer releases micro bubbles, that is again picked up by the return pump. There I added some filter wool on top of the return pump to catch the bubbles. Problem sorted.
Another test you can do, put your return pump directly in your display. Keep it halfway up the tank, so that it does not suck anything directly from the bottom. Keep it sideways. Like one big power head. Run it for a fe minutes and see if the stuff comes back. If it does, then you know you got fine particles in your display. Solution could be to mount a filter sock in front of a powerhead so that it blows into the sock. But pump inlet should be outside the sock. For a day, and it should catch up with a lot of the fine particles.