A water treatment device and methods of treating water such as cooling tower water, agricultural water, water used in the production of oil and/or gas, swimming pool water, and hot tub or spa water, are described. The water treatment device utilizes ultraviolet radiation, a magnetic field, and ozone fortified air to treat the water, typically resulting in reduced microbial contamination and reduced alkalinity in cooling tower water. Cooling tower water may consequently be run at higher cycles of concentration while reducing or eliminating deposition of minerals on cooling tower components. Swimming pool water and hot tub water treated with the water treatment device typically requires less chlorine, and chlorine levels are typically more stable than without the device.