Hi Jason,
I use sphagnum moss in my nest boxes and I'm pretty happy with it. It does NOT however hold moisture very well, although if you made it really sodden it might. I use it (in addition to its capacity as nests) to release moisture into the enclosure environment so I spray the nests down well in the morning when I check for eggs, and again in the evening when I get home from work and the geckos start waking up. Since it does dry out during the day, I figure it helps the humidity in their cages. Hope that helps.
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Terri
4:8:5 crested geckos: Popcorn, Goldfish, Harley, Bear, Big Red, Olive, Smudge, Blackie, Belle, Brilliant, Tina, Brick, Avis & Subtle, Toni(y), Kermit & Zipper