A very important point to consider when putting heavy rockwork into any tank is points. Pressure points that is. Recently I had three cubes of slate approx 10 x 10 x 10 in a 75. I never weighed them or botherered to calculate their weight, but they were heavy rocks just to pick up, let alone place precisely in an aquarium. Fyi I can still deliver and set up a 210 by myself as long as no more than a few steps are involved. That includes loading, unloading and getting it onto the stand solo, so I'm not afraid of a bit of heavy lifting! A very rough guess would be about 125 lbs each. I had plastic light grid on the bottom to help spread the load, but the main thing was the rocks were resting on perfectly flat faces. A little point sticking out taking all that weight would be a recipe for a bottom blowout. Two considerations when doing any rockwork should be to start with a bare tank with no substrate. Substrate trapped under rockwork accumulates crud. Substrate trapped under rockwork too heavy or intricate to remove easily for cleaning is bad news over the long run, and if a critter digs substrate out from under a rock and it shifts, a catastrophe could result.. If you want to use light grid or acrylic sheet or whatever to spread the load and take the weight of any pressure points fine. Then build your structure solidly from the base up, making sure it's stable as you go. You can wedge smaller rocks under larger ones to keep them from rocking, or even use underwater epoxy to fill small spaces to add stability.