A method of automatically controlling chloramine concentration in a body of water contained in a reservoir includes: (a) determining residual chloramine concentration in a water sample obtained from the body of water; (b) determining at least one of the following when the residual chloramine concentration is below a predetermined target chloramine concentration level: (i) an average rate of change in total chlorine concentration; and (ii) an average rate of change in oxidation-reduction potential; and (c) automatically engaging a supply of ammonia and a supply of chlorine to add both ammonia and chlorine to the body of water at a weight ratio of chlorine to ammonia of 5:1 or less when the average rate of change in total chlorine concentration is below a set rate of change and/or the average rate of change in oxidation-reduction potential is above a set rate of change.