Depends on what neps you are using them for. If veitchii and other epiphytes, I use charcoal (~1cm) + coarse burnt earth + limestone. Same mix for northiana. Charcoal is supposed to be beneficial in the mix, but I think it also can cause your media to be unevenly wet, as it obviously does not absorb as much water as sphag. I think it is good as crocking; place a layer at the bottom of the pot just so that when the moss ages and starts to compact, it does not cause anaerobic conditions which may lead to roots rotting. My 2 sen...
