Wherever you get plants for the terrarium they should be washed off. You can even swish the leaves around in warm soapy water and rinse. Like Kyle, I set the vivariums up well ahead of time, which allows the plants to establish as well as dispersing and degrading any fertilizers or other chemicals.
This is just a personal preference, but while a lot of regular house plants are suitable for terrariums, there are many more interesting things available from other suppliers. Our sponsors here have some-- (T&C, Black Jungle. etc.); Glasshouse Works, Birdrock Tropicals are reliable and inexpensive sources. You can also find dwarf species that will stay more contained. I just enjoy growing unusual plants nearly as much as I enjoy the frogs.
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Patty
Lost River, Idaho
D. auratus blue
D. galactonotus
D. imitator
D. leukomelas
D. pumilio Bastimentos (soon)