As forementioned, cypress mulch is a great natural substrate to use. You can mist it to assist with your humidity levels, and it also looks appealing. Some also use aspen, but I would be careful using this, as it is quite drying and tanks are hard enough to maintain humidity. If you get aspen wet it molds pretty quickly. Personally, I prefer to use newspaper or papertowel for mine. It isn't particularily attractive, but it is sanitary, easy to clean, cheap to use, and you don't run any risk of impaction, unlike and loose substrates. Hope this helped