The Mesh Filter Media Bag - Medium Duty 18" x 30" is designed for holding mid to low weight pond biological media. Commonly found in waterfalls and other bio-filters, these bags are perfect for use with Bioballs, Biocubes, Koi Kubes, and even low quantities of lava rocks. The bag measures 18" x 30" and can comfortable hold about 2 cubic feet of bio media. Mesh surface also allows for easy handling and cleaning of pond filter media.