The substrate needs changing because the frog pees in it and excretes mucus all over it, leading to nitrate buildup, and possible fungal and bacterial growth. I guess someone could rinse it thoroughly, and then bake it the kill off anything that might grow in it, but being coconut husk, I think the baking required might just turn it to charcoal. If you had access to some intense uv setup you could possibly destroy anything growing in it that way, but it seems like a bit of a hassle to save 3 dollars a month. Just use it as mulch.