I like eco-earth or any of the comparable products. It holds moisture well, is economical, and is a good medium for them to lay their eggs, when they get older. I used coco-soft as well with good results for ones that weren't gravid.
I've been using cocoa fiber stuff. It holds the moisture well, but I'm having an issue with it. There are gnats in the tubs. There are no gnats anywhere else in our house. I don't know if it was in the bedding before I got it? I just want them to go away. They fly out whenever I open the tubs.
Could try giving them a bit more ventilation. I'll get them from time to time in some of my leaftail setups... More often than not in the converted aquariums... which don't have as much ventilation.