A water treatment apparatus has a tank that contains a resin bed through which water flows. The apparatus is controlled by occasionally measuring the conductivity of the resin bed at two spaced apart locations to produce first and second conductivity measurements. The ratio of the two conductivity measurements is calculated periodically. The present ratio value, along with the minimum and maximum ratio values that occurred since a previous regeneration of the resin bed, are used to determine the movement of an exhaustion front through the resin bed. A probability of exhaustion also is determined and when that probability exceeds a given value, regeneration of the resin bed is initiated. The conductivity measurements are used to determine if the resin bed has lost efficiency and in which event a warning signal is sent to the user. During regeneration, the conductivity at one of the locations in the resin bed is monitored to detect whether a regenerant has entered the tank; if not another warning signal is sent to the user.