I'm not sure about toxicity, but it would be incredibly hard to keep it clean. I've tried to go with a natural set-up and I have seen many amazing pictures of some. There was one in the enclosure photo gallery that was unreal about a year ago. I've just gone with a tile floor with pieces of slate and some air plants. The air plants aren't glued down so they are really easy to keep clean and free of bugs. Leopard geckos aren't like dart frogs where their excrement can just go into the earth and degreate, they need a clean environment in order to thrive. They are however extremly interesting little creatures and well worth the work. Good luck and post pictures, I love natural looking set-ups.
Lisa
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0.1 Hypo Leopard Gecko
0.1 Albino Leopard Gecko
1.0 Blizzard Leopard Gecko
1.0 Albino Pacman Frog
1.0 Lovebird (Tony)
1.0 Elkhound Cross (Otto)
0.1 Malamute Cross (Paris)
6.0 Bettas