Galapagos Substrate Barrier is made from non-toxic mesh and measures 18in x 36in. Our Substrate Barrier can be cut or folded to fit the bottom of your terrarium. A Substrate Barrier is used to separate your substrate and bedding from the false bottom in a bioactive vivarium. The Substrate Barrier should be placed on top of the false bottom and below the substrate. Use the Galapagos Substrate Barrier in conjunction with a false bottom such as lava rock or clay pebbles, Bioactive Soil, Sphagnum moss, and Magnolia Leaf litter. Before adding substrate on top of your Substrate Barrier, make sure that the edges of the mesh are snug to the sides of the enclosure. If the edges of the mesh are not snug to the sides of the tank, the substrate can fall into the false bottom which can result in anaerobic bacteria that creates unhealthy conditions for both your plants and animals.