Loose, naturalistic substrates have many benefits for exotic pets. As well as encouraging the natural digging instincts that many animals have, it also holds humidity and heat well. If you want to have live plants in your enclosure, it's good for that, too!
Zoo Med Creatures™ Creature Soil is a safe, proprietary blend of natural materials, including peat moss, soil, sand, and carbon. This small, 1qt bag is great for a variety of creepy-crawlers such as spiders (including tarantulas), millipedes, insects, and other invertebrates.
