That is not normal! It's like a algae turf scrubber for bristleworms! There can't have been any appreciable flow happening there. Must have been feeding the bejeezus out of that tank.
Ummm hows that even happen? Why are they all concentrated in the overflow Pipe like that. Don't get me wrong I understand how biological populations explode due to an excess of food or lack of predator but Jesus! Did they all just reproduce and exist right in that Pipe!? Do u have link to original thread?