With crested geckos you can use quite a few substrates that are safe. Cypress mulch, coir (bed-a-beast), soil (untreated) or even leaf litter (recommend baking this first, to kill any bugs etc if you collect it from the wild). The various safe for reptile forest/jungle mixes work as well.

What you want to avoid is anything made from cedar or pine, as the oils/smells can be toxic. Also avoid small particles that are small enough to swallow but too big to pass through its digestive tract.
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PHLdyPayne

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