My idea is to rescape the tank and go with a temple/mayan/indiana jones type tank. That way the skulls would be more incorporated into the tank and not a distraction. I am issues coming up with some ideas. So people, please send some my way!!!!! FYI - 210 gallon tank, have lots of room to play.
I can't remember if it was on here or not, but someone had done a tank background with some meso-american styled decorations (I think it may have been Maya-inspired, but not certain) thought it looked pretty well.
I'd say get some styrofoam (wouldn't even have to be thick, and inch or 2 would probably be sufficient) - home insulation would be cheaper, but the floral stuff in craft stores would be easier to work with. look up some pics of mayan carvings and wall decorations and stuff, maybe pull a book out of the library.
If you have some artistic ability, you may be able to just mimic the style and do your own art, otherwise you could find a design you like, and copy it onto the styrofoam, carve it up, and coat it with something to make it look like rock (dendroboard folk seem to have good luck with drylock, I'm personally fond of an epoxy-sand mix, but that might not work if your carving is detailed)
Maybe something like some knapped arrowheads/knives might look neat, and since they are typically flint, obsidian, or glass, should be tank safe (aside from the sharp edges, but you could either dull them with something abrasive, or run a bit of epoxy or silicone or something on the edge)