Well, to be honest with you I have no idea whether a SSB will be OK over the long run of not. I've heard it argued that a SSB will cause problems earlier than a DSB, though. If you're going to rely on heavy skimming and flow to keep your tank clean, then I'd say definitely leave the sand in the sandbox (or at the beach). Have you seen that thread someone posted where they glued a layer of sand to the top of their starboard to make it look like a sandbed? Might be worth considering if you can't live without the look.