Hi Dick
I had that same problem when i just placed media in the bottom tray what happen was the water that return to the pond only exits the tray one side thus part of the media becomes submerged. This submerge media acts to trap the fines that were not filtered on this first run but what i have found was exactly what daisuke said to mike when using BH submerged the fine detritus will be eaten up by the bacteria on the second time round the filter or third time round thus the fast flow recommended. I don't see any fine sediment in the pond after i raised the media from my bottom tray. I moved all the media to the upper three trays and found that the filter worked so much better and water became even clearer in a week. Hope my experience helps you guys
tewa