I use acrylic rod and a drill bit to stack up my rocks. On my 90 I was able to use the Great Stuff Foam to fill in where the rocks met after drilling and pegging with the rod, I also threw on some crushed coral and sand to help the foam blend in. On my Finnex at work I used just the acrylic rod and the 2littleFishied putty - I don't like the 2 part putty. I was under the impression that it would stick rocks together - I either did it wrong or it ain't working. The putty did dry and seems somewhat useful in shoring up the rock pieces together. You can look in the nano section under my thread for the Finnex and see my rockscape using this technique.