I don't know how efficient this would be but it sounds cool in my brain:
Basically you are suspending rock formations in your tank. You would have to drill the roccks and put PVC or some rod through them to hold them up. I would say multiple rods for each structure. Then at the top either connect all the rods to a sheet of plexi or acrylic and lay it on top of tank. Or glue/drill it to wood and do the same.
Pic:

Basically you are suspending rock formations in your tank. You would have to drill the roccks and put PVC or some rod through them to hold them up. I would say multiple rods for each structure. Then at the top either connect all the rods to a sheet of plexi or acrylic and lay it on top of tank. Or glue/drill it to wood and do the same.
Pic:
