Aspen bedding is ok, but stay far away from Cedar and Pine. I find the best substrate for tree frogs, and most amphibians for that matter, is coconut husk. You can find this at pet stores as a compressed brick under names like plantation soil and eco earth. I get it at my local pet store for like $7.99 for 3 bricks. Each brick makes about 8 liters of soil. You just soak a brick in water and it expands. It holds moisture well, so it's great for keeping up the humidity for your tropical species. It also looks really nice and also makes a great potting soil so you can plant your plants directly into it.