bio-active drainage layer

[akane]

Not as important in my desert tank which may not even need a drainage layer but I am starting a damper tank heavier on the compost for a Dekay's brown snake. I see almost everyone uses hydroballs plus a mesh cover. Is the separating cover necessary when they are under several inches of palm compost and sand plus leaf litter? Would crushed coral or crushed lava rock work just fine since I already have these things (would have to crush the lava rock smaller) and they will be so buried? It doesn't seem like the hydroballs would have as important of impact on the humidity compared to those planting straight into them and that I just need somewhere for excess water to settle out if necessary.