A filter for a spa or pool, having a filter body which receives a filter bag having a sheath encompassing the exterior of the filter body and a pocket extending inwardly from the front end of the sheath into the filter. Operation of the spa water circulation system draws spa water into the filter bag pocket while concurrently spa water is drawn transversely through the filter bag sheath and auxiliary spa water openings into the filter body. An O-ring encircles the upper portion of the filter bag to releasably retain such bag to the filter body during operation of the spa water circulation system.