Bullfrog Replacement Cartridge filter for A, R, and X series from 2013 to current. The filter contains microban antimicrobial material. It is 50 sqft. The bottom has a 2 1/4 inch mail threaded end and the top is closed. Bullfrog filters are manufactured to provide excellent filtration. They are constructed to allow proper water flow for the Bullfrog Wellspring Filtration system. Bullfrog filters last longer, allow more time between cleanings and are easier to clean and maintain than other imitations.