in my poludarium, i am using the aquarium rocks that were in the tank already. I do not recommend this! mine is just a quick little set-up until i have enough money to build a real one. I've been told MANY times that eco-earth is the best choice. and also that you shouldn't use substrate that is 100% coco-earth.You could also use peat moss, which i might use if my upgrade. Just make sure it's safe if the frog accidently swollows it. Unfortunately, if one of my frogs swollows a rock, it's most likely going to be fatal. So i'm trying to find some land moss or something to cover the rocks. Any ideas what i should use?