CrystalClear Polish AC is bituminous coal that has been exposed to extreme heat, which activates the carbon giving it powerful absorbing powers. The billions of pores within the carbon act as hundreds of sponges soaking in many pond pollutants such as tannins, ammonia, nitrite and nitrate. The result is a polished, healthy pond environment. Replace bag with new Polish AC activated carbon every 3 months. It is important to maintain fresh activated carbon especially throughout the spring and summer season when pond pollutants are at their highest levels.