wow, I'm a perlite fan, I love using it in my terrarium, but I never flood my terrarium due to my irrigational system I have so the still water will collect on the bottom, I'm working on adding a pump, so dat the still water will be pump back up and automist every 30 mins and so, but using sand in my terrarium is hard, because the sand is too fine, as in not "coarse" leaving the sand to go all the way to the bottom, perlite floats and stays on top. My layers in the terrarium cosist of rocks, pea gravel, activated charcoal, lf sphagnum, and my potting mix, if my potting mix will be sand, the sand sinks through the cracks of the rocks and destroys my drainage layers. and to Dave S. hm...i'm worried a little about my health, I breathe in so much perlite dust when I'm making my potting mix.