As to your original question I use egg crate (light diffuser) inside the tank for all my tanks with sand. Its primary purpose contrary to popular myth is to help keep your rocks from sliding around which potentially could cause a rock slide. Over time the pressure of rocks stacked on top of each other and shifting sand has a tendency to move unsecured rocks around. The egg crate prevents that from happening ad the protrusions sticking out from the rock lock into the grid of the egg crate. The egg crate also offers some protection against falling rocks during aquascaping, but that is not its primary purpose. There are rumors that egg crate traps detritus, also not true provided you have some type of clean up crew that burrows into the sand like Nasarius Snails. I've taken down a few of my tanks that have been set up for over 5 years and I can assure you the sand in and around the egg crate was just as clean as the sand out away from the egg crate. IMHO its cheap insurance against rock slides.