after reading about deep sand beds on wet web media's website it looks like they want a minimum of 3" 4 to 6 is better. and that you can expect fine aragonite to dissolve at a rate of abut 50% in 24 months. I've decided to try this, 3" of silica sand (3.5" denitrates my freshwater tank and never dissolves) and top up as I can afford it with the fine aragonite sand. I am stacking my rock on 6" perforated tubes to keep the pile stable and to allow more water flow around the rock and to show how deep the sand bed is. I'll update my findings sometime in the future.
it has been said that silica sand is too rough for the critters but in my FW
tank the venustus' and frontosa's eat it to get the algae that grows in it and poops out sand turds (sorry) so I think it must not be all that sharp or damaging my pleco's belly and fins are fairing quite well also. the silica sand I bought was Walmart's play sand. It was very dirty when poured into the fishtank I think I'll use poolfilter sand this time I figure it should be more uniform and less dirty (we shall see)