I have a 7 inch deep remote deep sand bed in a 32 gallon bin with rocks on top and no lights.
I also have a bare chaeto refugium.
I installed the sand bed years ago to help with a nitrate problem it didn't do much but I never took it out. The top inch or two is productive.
The problem is the deep sand won't do much since nothing delivers flow down there except diffusion( molecular equilibration) which is very weak and slow. So since there is no organic carbon and/or much nitrate or phospahte in the depths , there are few if any of the facultative heterotrophic bacteria that use these things .
The piled rocks help a bit since when the current hits them it casues a pressure drop in their footprint which resluts in an upwelling and consequent downward flow to fill the void .This is called advection; natural reefs work this way. But even advection is too weak to get the bed working down deep. Most of the denitriication is going to occur in the top inch or less. So, if I built anothersand bed it would be shallow with a alrger surface area.
If I had to pick one or the other and didn't mind lighting it, I'd go with chaeto. It uses CO2, provides O2, consumes PO4 and NO3 and is relatively easy to harvest and keep clean in a bare bottom set up.
