RHX 50 Pcs Aquarium Pond Bio Balls 19mm Canister Filter Media Marine Reef Sump Fish The bio-sponge inside provide more surface area for growth of beneficial Bio-Spira bacteria BIO-Balls do not require replacement unless damaged or destroyed In the event of excessive buildup, you may gently rinse with water from an established aquarium, if desired Diameter of bio ball: 16mm. Includes: 50 pieces of 16mm Bio Balls