Heh, actually the opposite is true, they should be resting on the glass. If they rest on the sand they'll "settle" over time and risk toppling. If they're on the glass they have a stable platform and wont risk collapsing and killing your prized corals/fish/inverts. Cause its never the rock that has the $5 xenia on it that goes upsideown in the sand, its always the one with the rare zooanthid or rare $100+ acro frag.