Large volume tanks with sand beds give tonnes of micrscopic life.
If I sit and stare at the tank edge I can see lots of pods up againbst the tank edge.
If I stir up a an area of sand the size of a bread and butter plate, my fish get a big feed.
Sand is paramount in my opinion.
The ecosystem alone just in the sand supports the rest of the tank.
I believe the sand holds more goodies than the rock by 500%.