I was wondering what to use to separate the sand from the pea gravel. I ended up using nothing. The pea gravel is 10 mm and the sand I ended up with is 10-20. The photos show good stratification in a test jar- at least initially - even after gentle agitation. I'm encouraged. And happy I don't need some synthetic material in my filter.
sand_in_jar_zps157073c7.jpg Photo by Greg_M_ | Photobucket
sand_in_jar_closeup_zpsfda669e2.jpg Photo by Greg_M_ | Photobucket