The Exo Terra BioDrain Terrarium Draining Mesh is part of the BioDrain Terrarium Draining System. The non-toxic mesh will keep the underlying Draining Substrate separated from the decorative terrarium top-layer substrate. It also allows proper water drainage when used in combination with the BioDrain Terrarium Draining Substrate, as the BioDrain Terrarium Draining Mesh prevents contamination of the water by decorative substrate particles. It is ideal to create water parts and a biological filtration system in the terrarium using the Exo Terra BioDrain Terrarium Draining Substrate.