Nitrate reduction comes from the depth of the sandbed, not the size of the sand its growing in. NNR
comes from anaerobic bacteria, so the dsb
needs to be deep enough for it to grow. CC
is advised against because it allows big chunks of detritus to work its way down where it can't be eaten by detrivores and then rot. Small sand keeps it up at the surface where snails, crabs, etc can get to it.