I use organic top soil and sphagnum moss on top of the top soil in my tanks. I have dumpys, red eyes and grays... It is what they recomend for live vivaruims...

Furnish your terrarium with a good substrate – an inch or two (2.5-5 cm) of additive-free top soil is the best option because top soil contains natural, helpful bacteria which can better stabilise the waste your frogs will produce in between cleanings. Alternatively you can use coconut fibre (sold as bed-a-beast, terrarium soil, and other names). Coconut fibre does not have the balance of natural bacteria that is found in top soil and tends to sour sooner.