It's an old 55 gal. that I had lying around. I was going to sell it, but this idea came to me before it sold. I'm still searching for a computer fan, or a desktop fan so I can improve the air circulation!
I need to find a big tank like yours, I'm using a 10 gal tank as a hospital for some of my smaller orchids but a larger one would really work out better.
It's a thin layer of terrarium grade Cypress mulch used for lizards, frogs, and what not! I used to keep tree frogs, and I never had a problem with bugs or anything using the stuff.
So I figured I'd use it as a cushion between the pots, and the glass. It's also a lot cheaper, easier, and cleaner than changing out activated carbon, or rocks!