If you get some cheap slate tile like mine ($1.00 a Square foot), they are pretty thin and don't weigh much. Some people who have glass tanks also put a few layers of paper towels between the tile and glass, so the tiles wont scratch the glass or create a pressure point from the uneven tile bottoms.
Your tank is probably meant to hold X amount of gallons of water plus sand/rocks the slate tiles will weigh much less then if it was filled with water and sand/rocks.
-Justin