right now, they are balanced. We have pretty sturdy floors, the tanks don't vibrate much when we walk past. My husband has gotten really good at balancing rocks, and can usually do it so they never collapse.
I will admit, I was really worried about that at first until he walked past the tank a few times and nothing moved.
I don't think that I would want to silicone it together, just in case the fish found a new hole and the rocks needed to be adjusted.
This was all my husbands idea. I helped keep an eye out for holes as he was putting it together, but he thought of this. It was either this or a bunch of anachris. This was free, and seems to be working much better.