A method of monitoring residual treating agent in treated water wherein fluorescent tracers are used to determine the concentration of treating agent in the water at two different treating agent dosages and the difference in the measured concentrations of the fluorescent tracer at the respective doses are correlated with the residual concentration of the treating agent. The fluorescence response at the different treating agent dosages is used to determine whether the system is overdosed or underdosed on a continuous basis and to control treating agent dose accordingly.