Mike 47517-
I currently have black sand in my tank. The waste does sit on the top of the gravel and doesn't seem to sink beneath the surface. However, this does make it difficult to syphon, because your instinct tells you to syphon all the way through to the glass, and that sucks up too much of the sand. Syphoning just the surface of the sand can be difficult, and it will still suck sand into the syphon regardless. I know this dilema.
I often just gently sitr up the surface of the sand to get the waste moving and then net it with a brine shrimp net. I takes more time though because brine shrimp nets are tiny.
What I would suggest is to mix the sand with another type of gravel. I am about to do that with my black sand now because I want to be able to syphon down through without sucking up too much sand. Sand can be a pain.