I set up egg crate to cover most of the bottom of my tank, but I set it up on 1" diameter pvc pipe legs so it sits just above the sand. Better water flow around the entire rock. More sand for critters that use the sand. Small fish, stars, shrimp can get under the rock but big fish can't. Then about half of the tank (from the middle to one end) I did a second shelf about 3-4" above the bottom egg crate, but it only extends out about half as far from the back as the bottom egg crate. That creates a cave behind the rock for sleeping ect. I have rock piled up from 5-6" off the front glass (open sand) and it slopes up about 75% of the back glass. Rocks cover 99% of the eggcrate, including the edges and coraline hides the rest after about 3 months. It looks like 300 pounds of rock work, but there is only about 150 pounds in the DT and more in the sump.