Mostest important thing 'bout deep sand is to leave it undisturbed by human activity. MTS burrow shallow and help mulm work its way into sand and eventual it becomes fertilizer which is good.
Rooted plants bring entire communities of aerobic microbes down into anaerobic and anoxic regions as plants must pipe O2 down roots to keep them alive and they leak a tiny bit. These root-microbe communities eat up much toxins created by various oxygenless metabolisms and plants have access to iron, manganese, molybdenum which become free in anoxic zones and can penetrate roots before oxidizing again.
The bacteria, archeabacteria, and other microbes in the sand live in a complex balance which is safe, despite the occasion stinky bubble which might escape. When you dig around in it deeply, you may release toxins which would otherwise be eaten before escaping into water. So, try not to uproot deep roots but cut them and leave them in. Plant carefully using tongs or tweezers made for nonintrusive planting.
The sand will develop into a rich, fertile substrate over time and act as a powerful biological filter and possibly supplement CO2 levels. At depth it can even denitrate NO3.
When the flourite runs out of ummph, the sand will have developed an adequate store of nutrients.
HOWEVER, if you intend to disrupt the sand a bunch, be consistent. Don't allow decay to build up deep and THEN disrupt it. Such an approach is messy and annoying to tank inhabitants I am sure.
Coarser sand is preferable. I suggest meshed sand ranging from ~0.2mm to ~1.1mm as it permits rapid diffusion of nutrients, oxygen, and other chemicals through sand bed and speeds cycling of a deep sand tank considerable plus mulm just sinks into it and becomes plant food.
