Thank you! I did make them. Very simple to do, too!
1) Get river rocks. ($1 at Dollar Tree.) Clean with hot water, then soak them overnight.
2) Get aquarium-safe silicone sealant. (The Loctite brand is $4.50 at Lowe's.)
3) Stack rocks until you're happy with the arrangement.
4) Use the silicone to 'glue' everything together. Let sit at least 24 hrs to dry. Rinse with warm water.
5) Put in tank.
In the back of the tank (behind the mesh floater), I also have a terracotta pot attached to a silicone suction cup. My guy loves it, but I noticed today that the silicone adhesive was starting to loosen from the suction cup. I'll need to work on finding a better way to set this up, since the little guy likes hanging out in there.
I've also stacked colored glass to make ornaments too.