The Exo Terra® Bio Drain Terrarium Draining Mesh is part of the Bio Drain Terrarium Draining System. The non-toxic mesh will keep the underlying Bio Drain Draining Substrate separated from the decorative top-layer substrate. It will prevent substrate particles from contaminating the water while still allowing proper water drainage. In combination with the Bio Drain Substrate you can create water parts and a biological filtration system in the terrarium. The clean terrarium water can then be circulated in Waterfalls, Cascades or Dripping Plants.