Abstract: A self washing water storage tank has a floor, a cylindrical, generally upright, peripheral concrete wall which sits on the floor and extends around and defines a water storage chamber, and a concrete roof that is mounted atop the wall in substantial covering relationship to the chamber. The roof is essentially flat and has an internal surface which faces the chamber. The tank is arranged such that during the fill cycle the water level rises to a high water level which is above the interior surface of the roof so that such surface is washed during each fill cycle to prevent damage from corrosive gases which may escape from the water and accumulate on the interior surfaces of the tank above the water level.