easy, easy, some people put egg crates under their gravel but usually with african cichlids. African cichlids do a lot of digging and therefore, sometimes causing rocks to fall and the egg crate protects the rock from hitting the bottom of the tank. You do not need to do this with saltwater if you are using live sand. Some members i seen using egg crate if they go bare bottom. Hope this helps. Happy reef keeping.
Can't the use of egg crate under the sand be a form of anoxic filtration. If there were a dead space between a deep sand bed and the tank bottom it will enventually create an anoxic filtration system. Just some form of biolocical filtration I read somewhere.
Your base rock should be placed directly on the bottom of the tank and then the sandbed should be added.
The idea behind this is so your rockwork is supported from the very bottom and will not be able to collapse in the event that fish or other reef creatures burrow beneath the rocks.
No need for eggcrate