I've used two substrates over the years and have never had any problems. Expanded cocoa fiber, as mentioned above, is what I use with all young Ceratophrys. There is literally no risk of impaction. However, it can get expensive if you want a deep substrate large enclosures, especially if you are cleaning frequently. Therefore I have also used standard topsoil with no added chemicals or fertilizers. One cubic yard is less than $2 so you get a lot of substrate for little cost. This however needs to be sifted/checked for pebbles. I also back pan fulls of the topsoil in the oven at 300 degrees for 30 minutes inorder to kill any microorganisms that may cause problems for the frog. If you follow those two steps topsoil can be a very nice inexpensive substrate.