Water quality problems in potable water storage tanks include temperature stratification, stagnation, dead zones and short-circuiting. These problems stem from the simple fact that water in reservoirs forms thin horizontal layers due to differences in temperature and pressure. One solution with proven results is to mix the tank thoroughly, from tank floor to water surface. Well-mixed water delivers consistent sampling, consistent water age and consistent water quality. Understanding why water layering occurs in storage tanks will help utility managers determine how best to ensure that the water their customers receive is of the highest quality.