I hadn't thought of that. That is an interesting idea. It is a mulitiport: Filter-Rinse-Backwash-Recirculate-Waste-Closed I don't know the brand off hand--I'll check tonight.

The part that confuses me is that the water is other wise crystal clear--all other signs would appears that the filter works just fine. I took it apart last night and examined the grids--no problems at all. However, only the top two third of the filter were covered with DE. I cleaned everything and added more DE this time. Nevertheless I am with you that even without DE at all you would think particles that big would be filtered. That is why your multiport suggestion is interesting.