By simply looking through the old threads you'll find more problem threads about "Pebble" sized Caps vs. Sand Caps.
I feel the "seal" effect of sand is why it works. Don't let people that don't even keep dirtied tanks scare you with Anaerobic Apocalypse from compacted sand.
Just keep to reasonable depth levels 1.5 to 2" Dirt of your choice 1 to 1.5" Sand cap.
>>> Uniform sized sands:
Tan - Play Sand
Black -Black Diamond blasting grit.
>>> Irregular Sized Grains
Tan - Constuction grade, "all purpose" sand
They all work equally well.
PRS and Black Diamond are pretty clean out of the bag but, it al should be rinsed prior to use.
Good Luck with your tank