I think water is 8# per gallon, or close. Usually most people caution that falling rocks are more a reason than weight for leaks/breaks. You can use "eggcrate" found in the ceiling tile section of a hardware store to help dispuse the weight more evenly and guard against falling rocks. Secure the rocks by some type of aquarium safe adhesive ( I am drilling holes in mine and using PVC "pins" to lock them together along with silicone and epoxy cement)Also using foam insulation boards cut to fit under the tank can help with any "bowing" that occurs, if any , for an extra precaution. I would guess that the aquarium could bare the weight, if not you had way to muck rockage.

Aquarium glass are very strong. You should ensure to get the tank that has tempered glass, very hard to break and the strongest. Remember, the more rocks and decorations you put in, the less water there is. When you add a big rock, not only the weight of it, but the height and width (volume) of the rock can affect how much amount of water is reduced. Yes, the gravels greatly affect the amount of the water in the tank. If you have a 30 gallons tank, with all the gravels, plants, and decoration included, the amount of water would be around 23 gallons. So, do not just follow the tank gallons as said on the box, you would have to calculate all the factors.