I wonder, I doubt it would work...but there is a very outside chance. I was thinking, if that flake spray paint would be safe in a tank (I doubt it, but worth a shot) then you could get a piece of PVC pipe (like a 4-5in diameter) cut in half, and paint it with the flake paint. This could give you a start of a cave.
Like I said, probably cant put the paint in the tank...but hey, its an idea |