So, there're two common methods of dealing with draining in bioactive enclosures: installing a bulkhead (which can be risky with glass enclosures if you're not skilled with a drill!), or providing some other form of access to the water table. It didn't take me long to rule out bulkheads for me, so I havent done a lot of research on how to make it work, but I'll share what I plan on doing (yay graveyard shift doodles!!

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Essentially, I'm going to take a PVC pipe (or similar), drill some small holes in it that will allow water through but not the hydroballs/drainage material, and place it in one of the corners of my viv. Then I'll build my substrate layers (hydroballs, barrier, soil, leaf litter) around it, and cap it off with something so that feeders and CuC can't get lost in there. That'll allow me to siphon off excess water with a baster or similar if the water table gets too high. A bit of water isn't bad, though, and can be beneficial as it carries a lot of nutrient run off! It's stagnation that can be harmful. I may skip the barrier layer to allow roots to dip right into the "nutrient soup", but haven't made a final decision yet.
Hope that helps! I'm still doing some reading on the matter, but these are my plans so far.