For helping to hold the smooth rocks in place I used a 2 part putty type epoxy from fuvel. It is also used for building salt reefs, and for anchoring corals to the rock. Took a bit to figure out how to work with it, but it works great for holding the rocks. I used it for the rocks in my crayfish tank, as well to make the pool for my salamander. The only suggestion is to look for one that dries black.
the down side the that is the rock are permanent. which is fine if you are 100% set on hardscape, unfortunately, some of us never are....lol
i like to use permatex brand 2 part epoxies, ge silicone ii, and good old fashioned super glue to hold stuff together in my tanks and vivs. i have seen that stuff for corals and sort of dismissed it.
does it hold well? i mean, if i were to try to move something that was epoxied, does it take much force to break it free?