High surface area provides optimal environment for beneficial bacterial colonization. With all of the surface area but, 25% less weight than lava rock and half the weight of gravel, expanded clay Bio-Rocks are the more stable, less abrasive alternative to heavyweight bio-media. Expanded clay is a lightweight, stable and less abrasive alternative to lava rock and gravel. Bio-Rocks are durable and easy to clean. Use one box of Bio-Rocks per BG2234 Media Bag. Bio-Rocks have a 5-year warranty.