The original Arctic Spa silver sentinel filter. This one screws in, not to be confused with the one that drops in. Silver sentinel filters are better than paper pleated filters because the material is thicker and filters out much smaller particles and hold them in the material. Paper pleated filters ar thin and the holes in the media are larger so they dont trap as much small particles. The filter is made from recyclable material. Usually lasts 3-4 months. Our prices are very competitive.