The Hot Tub Bio-Filter by Spa Rocks utilizes volcanic rock to filter the spa water. This special water treatment media involves using natural volcanic glass which is ground into very small granules. These granules have a highly porous and abrasive structure that makes them excellent for trapping contaminants and sediment in the water filtration process. The volcanic rock is characterized by a high surface area and abrasive properties that enhance its effectiveness as a superior filtration medium.