Prompt Set Spas water filter cartridges help remove dirt, hair and other particles from your spas body of water. Regular maintenance of filters can provide clean and clear water. Filters should be cleaned after each usage and be replaced after each 7-10 days of use. Our filter cartridge measures 4-1/3-inch in length by 3-3/4-inch in diameter with a 2-inch open hole on top and bottom. Pack of 4 cartridges. Suitable for Prompt Set Classic and Deluxe Spa models. Fits Prompt Set Classic and Deluxe Inflatable Air Bubble Massage Hot Tub in Box.