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 Dripping Plants.What is the Exo Terra Bio Draining Terrarium Substrate?Exo Terra Bio Draining Terrarium Substrate is a naturally occurring non-organic gravel. By separating stones and soil it allows water from your reservoir to slowly enter the soil and air instead of being immediately absorbed.What are the benefits of the Exo Terra Bio Draining Terrarium Substrate?The Exo Terra Bio Draining Terrarium Substrate ensures that your soil does not become waterlogged, protecting your plant’s roots from rotting. Furthermore, it gives you more time to spend with your amphibian and reptilian friends by reducing the time spent on maintenance.Why is it important that a terrarium has proper drainage?Proper drainage is advisable in all types of terrarium as it catches excess water coming through the soil and keeps it moving to prevent stagnation, which can, in turn, lead to a build-up of harmful bacteria and strong odours.