If you are going to have a DSB
then you need "critters" to gently move the sand and process the waste that accumulates there. Without a proper population of sand infauna the DSB
will turn into a polluted clump of rotting organic matter. If you do not intend to have a DSB
then use no more than 1/2" of sand. If you have LR
the worms will migrate to the sand anyway so it shouldn't be an issue either way.