Hino,

I'm confused. Do you have sand under the screen? Only water should be under the screen in a plenum. That's the purpose of the screen, to create an anoxic cavity under the sand bed.

The purpose of the burrowing critters is to turn the sand over and eat any detritus that accumulates there. The critters aren't going to penetrate through the screen. I have a sand sifting star I will give you if you want it.