Also, no need for egg crate unless you have sharp pointed areas that can create pressure points on the glass. I've never used egg crate under boulders, I just ensured that the boulders were smooth/flat on the bottom.
When you place the boulders, do you put them right on the glass then fill in the sand around them? Or do you put the sand in first, then the rock work?
It’s for Frontosa, so I’m guessing they’ll dig a little bit.