I am envious of the 135 gallon
I tried someting similar with styrofoam back ground in a tank with Red devil ,Leporinus,and Jaguar. Like you I used stones at the bottom but the stones I ended up using were Very large and although they worked to help keep the background in place,, I did not forsee that the cichlids would begin pecking at the back ground and I soon ended up with all kinds of bits of styrofoam floating about in the tank.
I removed the background and painted the back glass black.
At first, the 3D was pleasing, but the fish simply began to tear hell out of it along with wrestling with the filter intakes when they weren't scuffling with each other.
I ended up decorating the tank with very large river stones and large driftwood pieces and the fish seemed to enjoy the enviornment.
I wish you the best on your new tank !