Hi Levy.
Your question in relation to placing ceramic tiles at the bottom of your tank for aesthetics is certainly acceptable. I thinks its a better way to go rather than using gravel or sand actually. So in my opinion i cant see why you shouldn't.
The only initial concern i had was for debris getting caught under there, but since you are going to seal all the edges around the tiles, i imagine that will keep water and any debris from getting caught underneath. Im thinking you will do this after the seals for the glass have set, and that you will take into consideration the ability to remove the tiles at a later stage if you wish to do so.
One more thing that comes to mind is different expansion rates of the glass and the tiles, so i would allow for large gaps and a flexible sealer to adhere the tiles with.
I look forward to seeing the finished product.
Good luck with you project.