Zoo Med's Naturalistic Terrarium Substrate Mesh is a non-toxic mesh designed for use between substrate layers. When used above Zoo Med HydroBalls, for example, it allows proper water drainage which keeps substrates from mixing and contaminating the water filtration system. Excellent for use with waterfall applications. Designed for specific compatibility with the Zoo Med NT-3 (18" x 18" x 18") and NT-4 (18" x 18" x 24") Naturalistic Terrariums, the mesh is also great for use in any terrarium environment. For larger spaces, simply combine multiple pieces.