[hllywd]

If you're worried about dropping something on the bottom of the tank, starboard, or even acrylic would be better ideas than the ceramic tile. As long as you use some reasonable guidelines like the previously mentioned spacing there shouldn't be any integrity issues. Triple check before you begin to drill whether the tank is tempered or not. Back to the ceramic idea, it will need to be notched around the bulkheads, I wasn't sure from you post what your intentions were... As for sealing it to the bottom, the previous owner of my tank siliconed acrylic to the bottom, it is not water tight and I assume that was his idea, so be forewarned...

Additionally I'd be very careful supporting the bottom as you intend, any undue upward pressure and you're likely to create problems you wouldn't have had otherwise. Finally most tank manufacturers recommend against using foam under there tanks.

Good Luck!
Tim