Exo Terra BioDrain Terrarium Draining Substrate is a natural, non-organic substrate that allows excellent drainage of the terrarium. It will not rot or decompose like other natural substrates. The optional BioDrain Terrarium Draining Mesh ( sold separately) will keep the BioDrain Terrarium Draining Substrate separated from the decorative terrarium top-layer substrate and prevent the different substrates from mixing and contaminating the water. The BioDrain Terrarium Draining Substrate will function as a biological filtration system. The clean terrarium water can then be circulated in waterfalls, the Exo Terra Rain System or plants.