Egg Crate Under Rockwork ?

[Aquattro]

I would never put rocks on sand again, I had a tower collapse as the sand shifted under it. Probably induced by some livestock, but it was a mess. Didn't hit glass, but from now on my rock sits on solid footing and sand goes in second.

[viperfish]

I stack mine very carefully, and a little bit of DD putty between them ensures nothing is going to move. I also place some putty anywhere a rock might touch the glass on the back to reduce the stress concentration.