For humidity and ability to repel mold, pure cypress mulch is the best. However, I find it pretty hard to locate anywhere near me at wholesale price (most is mixed with pine and other woods) and the stuff they sell at pet stores and from online pet suppliers is overpriced.
I go with Eco-Earth coconut fiber substrate, it's a bit more effort to prepare but it's pretty dang cheap and holds humidity well. Plus+ it's eco friendly, so that's always nice.
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