Crate On Bottom Of Tank?

[Tim A]

I have seen some pics online, just wondering why people put that egg crate or air duct diffusers on the bottom of the tank with a sand bed on top? Is this so frag plugs can sit easier? Or more stable for LR placement?

 

[Kyle1970]

Prevents cracking bottom pane of tank by distributing weight.
Helps to prevent rocks cracking the tank if the were to fall over

 

[shamous113]

The jagged edges of the rock catch in the voids of the egg crate which locks the rocks in place and helps keep them from shifting over time from the pressure of the rocks above making it more stable. It also stops my pistol shrimp and goby from removing every thing from under the rocks, i think they would have moved if i didn't have it. it was a pain to put sand in after rocks but worth in the long run. I also used it as a plenum on the dsb in my sump, by covering it with very fine non-metallic mesh will keep sand, dirt and critters out of there.