I have stacked them, Very Flat, Heavy pieces on the bottom...
IT is a VERY Solid structure, No Epoxy was needed, but I will have a pistol shrimp, so... there is some Epoxy, Here and There. Only in a few spots tho', to help hold it together over time...
One step ahead of yah' also. Complete Room on the back, for cleaning, and water movement, '' No dead spots''
I still have to put some small rocks here and there to fill in the ''KnifeRockPile'', to hide the plumbing a lil' better. Lol I think I may do a Very Thin, Skinny, Tall pillar in the back middle, to add more depth.. Here's and Update shot... . and no reason to use a Chisel on this one. I got Tons of Rubble,lol.