this has been discussed before, and I think the reason for a lack of responses is because, in general, there is a bit of a lack of interest in doing this. Not because it's not a nifty idea, but because it's better done as a showcase for a few minites, than a long-term decoration. The fact is, most people on this site will want lots of flow in their tanks to adequately disperse nutrients/Co2, and this, in turn will also disperse whatever is going through the "fall." The ways to get around this would be:
A: Something going through the fall that was heavy enough to not be pushed around by the normal flow in the tank. You'd need a much stronger pull than air to get this up and over the fall, and it would in turn fall much faster than sand, hence, ruining the effect of the gentle waterfall.
B: Enlarge the "pool" at the bottom which collected the sand to return it to your uplift tube. Too large, and this can be unsightly, to say the least.
C: Reduce the flow in the rest of the aquarium to avoid the sand spilling out. Cardinal sin in most books around here
I'm not sure if that answered your question at ALL, and for that I apologise. I haven't tried this myself. As far as this particular application, you might be one of only a few people here actively trying at it, and for that you are to be commended. Most people (myself included) won't go into a realm already known to be a potential headache, and I'm eager to see if you make it past that to implement this as a constant decoration in your aquarium.