I prefer thin layer of substrate because it’s looks way better and I love it how the rays act in it, always sifting though it etc and they love to bury themselves for a while after they’ve had a good feed. As regards filteration, sand may make a good filter media inside a filter with water flowing through it but I’m not convinced it adds any benefits of filtration just sat on the bottom of the tank.
What about gravel, small rocks and stones? They add as filter media as well no?
I'm planning to keep Florida gars, Dorado, tiger shovelnose catfish and a motoro ray.
I would prefer having sand and some gravel along with driftwood and some rocks.
But as of now, I'm planning to go bare bottom but with rocks and driftwood.