From what I understand adding DE to a sand filter creates a small layer at the top of the sand and catches all the fine particles. That means it is also catching anything bigger. Is there anything left for the sand to do? It seems to me the ideal thing to do would be adding sand to the filter until get you above the laterals, thin layer of DE, and fill it the rest of the way with sand. That way the sand gets the big stuff the DE gets the little stuff. I am guessing the DE will stay put when you backwash or it wouldn’t stay put during normal filtering.