A filter assembly for filtering water in spas, swimming pools, hot tubs and whirlpools, having a first or outer filter element, a second or inner filter element removably installed within the outer filter element, a first coupling member associated with the outer filter element, a second coupling member associated with the inner filter element, the first and second coupling members engaging one another to connect the inner filter element with the outer filter element, and a releasable detent arrangement resisting disengagement of the first and second coupling members from one another. The outer filter element includes a filter medium for mechanically removing particulates from a fluid to be treated and the inner filter element includes a filter medium containing fluid purifying particles.