bits don't sink down into the sand, so there is no need to stick your gravel vac down into it. Instead, hover the tip of the vac just above the sand surface. The poop/food/gunk gets sucked up, and the sand stays.
Secondly, if you do want to mix up the sand, you can stick the vac in and pull it out, then gently shake the sand out.
Its a bit of a learning curve, and you'll always get *some* sand out. Typically on a 10 gallon water change on my 55g, I'll end up with about a tablespoon in the bottom of the bucket each time. 2 tbs of sand per waterchange.
I love the stuff, but it takes some getting used to.
