Method and apparatus for treating potable water in municipal and similar tanks to reduce and remove undesirable disinfectant byproducts such as trihalomethanes from the water by providing a water circulation system to create circulation patterns in the tank water and an air flow system for creating an air flow pattern in the headspace region of the enclosed tank above the water surface. In operation, a portion of the tank water is drawn-up a draft tube from the tank floor to above the water surface and sprayed through a nozzle outwardly about a vertical axis and slightly downwardly toward the surface of remaining tank water. The air flow system creates and directs a high volume of air through the tank above the water surface to volatize undesirable trihalomethanes in the drawn-up water portion to gaseous state to enter the air flow pattern and exit the tank into ambient air.