A water treatment insert for a pool skimmer system includes a chemical treatment portion separated from a mechanical filtration portion. The treatment device includes an upper portion and a lower portion. The upper section can include a vertical wall or walls, which should be substantially watertight so that substantially all the water entering the pool skimmer flows through the upper portion, and then into the lower section. The entire device is preferably received in the filter basket receiving section of a conventional pool skimmer, in place of the conventional filter basket. One of the upper and lower sections provides mechanical filtration and the other chemical treatment, to enhance the water treatment function of the skimmer.