For my tortoise, snails, and frog I use peat chunks. As Roy mentioned, a bonus in using this substrate is that you can safely feed on it, unlike most other loose substrates. Because it is so acidic, it resists mold as well.
Ever put peat under a microscope? I've seen more microscopic yeasts and fungi in peat than in most topsoils. And the types of mold peat does harbor tend to be the ones that produce a lot of mycotoxins. I see a lot of respiratory, skin and digestive tract infections caused by peat substrates.