[jk]

I have been using something called "bark mulch" for a little while. It's bark that has been shredded into small pieces. I haven't been using it long , but so far the plants are healthy. I think it's made of bark from fir trees, so perhaps it would be difficult to find in many tropical and subtropical locations. However, any other plant material in small pieces that decompose slowly when wet would be possible candidates for Nepenthes medium.