I dont know but I compared three tanks and it seems better if you add PVC
pieces and put rock on top and add sand..
Rock I beleive has to hit the glass or on pvc
because the sand on bottom I think wont come live..
three tanks i have noticed:
bare bottom Nitrites <5 ppm
Sand first Rock Last Nitrites around 10-20ppm
Rock on PVC
Nitrites 0 ppm
And all three tanks have same water, etc.. So I guess from my test ROCK ON PVC
is the way to go..