if your siphoning with just a hose it will suck up sand, if you use like 3/8 or 1/2 clear you can see sand in tube, block drain side with your finger and it will drop back down. you can control flow with your finger on drain side.
I bought a siphon tool from petsmart for the same problem. When i siphon the sand would get sucked up but wouldnt get sucked out from the tube, all i would have to do is light up the siphon from out of the water and the sand would drop down
if you have a vacuum typ where one end is bigger it will still suck lite sand, but just control it.