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Yes you can use the gravel vac. The good thing about sand is that fish waste is too big to filter through the sand and reach the bottom so you can just Hoover directly on the surface of the sand and pick up most of the waste that way.
When I have to vac my sand I try to hold the vaf at about a 45 degree angle. The sand should be heavy enough that it won't get sucked out for the most part.
If the debris is too heavy to be sucked up by the siphon, a trick I have discovered is to turn the siphon backwards so it it sucking with the small end. This smaller diameter gives more sucking power, and if you swirl it in circles above the sand the debris will make a little tornado and will be sucked up easily.