I'm noticing now, that after a cleaning the screen will appear to grow but will have white spots on the screen from pods, presumably from micro pods. As nutrients build up in the water, all of a sudden on about the 3rd day, the screen is covered in green the next morning, presumably because the growth was so fast (catching up with nutrients) that the micro pods could not keep up.
At least that's how the current size screen, with the current lighting, with the current feeding, is working. This of course does not explain why the same does would not occur in a waterfall. Or maybe it does, but the waterfalls or so much better operated that it's not noticed.