I've used a lot of different things, but am currently using an organic compost bought in bulk from a nursery. I cover it with small aquarium pebbles and lace it with Java moss, then let this grow awhile before introducing the frogs. Pastor Josh's solutions are also good. Most anything that doesn't contain fertilizers is all right. A layer of leaf litter, especially oak leaves seems to help. I had a few months of advocating cocoanut fiber, and while it works, I'm having second thoughts about it that I haven't figured out yet.
The Australian sphagnum no longer comes alive. It used to, but I suspect that import regulations now demand that it is "sterilized." I've had a lot of experience with this dating back to the 1950's. I used to use milled sphagnum mixed with sand to grow difficult seeds, such as rare cacti. It had a sort of antibiotic quality that prevented damping off and rot. In terrariums, it would indeed come alive, but eventually over-grow the entire set-up. I still use it, and it is useful, but it no longer "comes alive"-- The green stuff people may see on it is merely algae, now. It is still however, worth using, because it is a nice substrate, especially for stuffing behind cork bark for bromeliads to hold on to. I'm sorry I have no exact information about what occurred regarding the changes in the properties of the sphagnum, but am merely guessing.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus