I have one. Takes the sand from the bottom to the top using bubbles. Up the top is a thin sponge that releases the air but not the sand. Sand then falls out the front of the waterfall and down to the catching area at the bottom where is goes around again.
Issue is with gettting the sand and the fish eating the sand. I am just using it as a rock display now due to the above. Can take a couple of photos and post them if you like.