For a deep sand bed (DSB) to effectively complete the nitrogen cycle they need advection. This is accomplished with a medium to high flow over the sand bed. Random flow is best.

A vegetative fuge w/o a DSB can operate w very low flow.

A cup of live sand from a tank w/o sandbed predators and a small freshly cured piece of live rock will contain enough life to biodiversity to rapidly inoculate a sandbed with all the life a sandbed will need be self sustaining for a long time.

I do like to add some stomatella snails to a fuge as they will reproduce.