I have the same problem of my sand getting sucked up the syphon. I don't really care that im losing sand, but i cant dump it into my sink/toilet b/c it will settle in the bottom of the sumps or septic tank.. where ever it goes to... Next time i clean im going to rubberband a piece of nylon stocking to the end of my spyhon tube and see if that will catch it.
Someone told me buying a sphyhon with a longer uptake tube will also help but i havnt tried it yet