Jungle mix is a great pre-mix. Avoid potting soils with added fertilizer. The nitrates aren't good for the frogs and the terrarium plants, as someone said, are usually too enthusiastic to need it. I have also read somewhere that vermiculite (expanded mica) should be avoided as well, but there was no explanation why. I have the reptile bedding someone mentioned below in one tank, and it also works. The plants seem healthy in spite of the slow breakdown of this stuff and paucity of nutients. I use a lot of brown sphagnum around the bromeliads and to stuff cracks in the back panels. When fresh, it helps prevent root rot while the bromeliads are establishing and rooting to the panels and branches. It has some sort of "antibiotic" quality.
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Patty
Lost River, Idaho
D. auratus blue
D. galactonotus
D. imitator
D. leukomelas
D. pumilio Bastimentos (soon)