Abstract: A water treatment device f or use with evaporative coolers provides treatment of the water supply for such coolers. The device may be installed in series with the cooler in the water supply line and does not require any modification to the evaporative cooler itself. The device has a central housing having an interior divided by a longitudinal baffle, which provides a water inlet side and a water treatment side. A chemical insert, preferably formed to closely fit the interior of the treatment side, is installed in the treatment side of the device, and water is caused to flow from the water supply line to the inlet of the device, downward through the inlet side, and then upward through and past the chemical insert in order to provide maximum exposure of the surface of the insert to the water to be treated. The treated water then f lows through an outlet and then through to the evaporative cooler.