Thanks.
I cant think of a reliable way to do it without tearing down the tank, those things FLOAT. It wasnt enough to weight down the mid sized piece (on the bottom) with a slat tile siliconed, I had to place 2 fist-size rocks in the front and back.
I chose to use several rock modules instead of a whole-piece background for 2 reasons - usable space, and uniqueness. However I had to think it thoroughly (water circulation, hides vs. ease-to-catch, equipment placement) before sawing and sticking
and it took a while.