I was not really worried about the weight of the rocks on the glass, the set up is what i would call a "shell" if you will, the thick piece of wood coming out the top is what the rocks are lying against, there is no rocks under them. (all together 10 lava rocks on the outside)
I was most worried about the total weight on the floor, like i have always been, this tank is on the 2nd floor, all together the lave rock weights less than the river rock i have always had in the tank.