Deep sand bed methodology

[jcnkt_ellis]

so its better to put maybe 2 inches LR rubble below the sand bed, or 5 inches of sand, no rubble?

You need an anerobic zone (low oxygen content) for denitrifying purposes so, imo, 2 inches of LR rubble below the sand bed would destroy that zone. 5 inches of sand, no rubble, imo, would work best. The idea, as I understand it, is to keep from using too fine of a sand, or too deep of a bed, in order to prevent an anoxic zone (no oxygen content) from forming underneath the anerobic zone.