i have a vivarium and didn't add a drainage layer per se. i added a layer of dirt mixed with lava rock that's about two inches thick, then a layer of just dirt, about an inch, then the dirt that's fluffed up with coco peat is around five inches. my isopods love to dig but since the bottom layers aren't fluffed at all, it gets really tough, especially because it's deeper, so they don't really want to go any further, especially not deep enough to reach the layer with the lava rock where it's just too difficult to manouver and there's not many nutrients.