Yeah it all depends on how porous the rock is though... Remember, that the rock is probably full of holes so it will displace some water but some will also seep into it. Its tough to calculate, but usually the rock increases the weight of the tank. N1z, I believe you've got a 55 right? 55s are heavy, but not astronomically so and a modern house shouldnt have too much trouble supporting it especially if its near a load-bearing wall.