Thanks for the reply.

I'm not sure I can get rock fern, so I'll probably try the Boston fern and trim it back when needed, unless someone says it's dangerous for the frog.

How did you know your moss carried nematodes? Mine just sprang from the soil I bought, after a while, so it was probably there only as dormant spores. I doubt nematodes could have been in the spores, but since I don't know anything about nematodes, I'm not sure I can take this for granted. They seem to be microscopic..... Did the nematodes cause problems for your frogs?