I thought the filter aid also, maybe it got into the sand at the top couple inches or so and is causing it to plug up faster? What is a filter aid exactly? I can only imagine it is smaller than the sand, and that the purpose of it is to filter out smaller particles. This is probably the same theory as putting DE in the sand filter. Maybe you should remove an inch or so of sand from the top of the filter medium and see if that helps. It is a sand filter right?