Natural but non-organic substrate that promotes the flow of excess water in a terrarium.Unlike other natural substrates, this one will not rot or decompose and serve as a biological filtration system.It is part of the drainage for terrarium. Non-toxic, it separates the lower layer of drainage substrate from the upper layer of decorative substrate. It thus prevents substrate particles from contaminating the water while allowing adequate drainage of the water. Used with the Bio Drain substrate, it creates aquatic areas and a b..