Actually I've think decided not to use Viastone (which I have on-hand for other uses)(won't need weed barrier cloth either, which I have on-hand, for other uses), & I hate very much. Evil stuff! Maybe I'm wrong, but don't really see the need of it, especially the more I read of the Walstad method.(wish she'd do a 2020 edition).
Another concern of mine is the current way I've designed the tank is having the aquarium portion of the tank at approx. 30% (>1.25 cu. ft.,<10 gal. or 6> inch depth. The upper portion around 70% (2.5 cu.ft., +-20 gal. atmosphere.)
The Walstad way is heavily planted. Which I don't want. And for obvious reasons can't have. Plant selection will be critical. And as noted by DaveKS, I'll have a selection of air plants, but %wise a little more terrestrial plants (if you can count semi-epiphytes as somewhat terrestrial.)
What I'm getting at is: 1) will there be a leaching/run-off into the aquarium side & to what effect? 2) if so, how to alleviate any - issues and will a Viastone/leca bottom or buffer (of course separated by a physical barrier) provide any benefit?