Abstract: The invention relates to the control of pH in water, preferably pool/spa water along with the control and/or removal of algal populations, by adding to the water a desired synergistic amount of a lanthanide-containing compound, a transition metal salt ions sufficient to bind with hydroxyl into a slightly soluble or insoluble reaction product, thereby removing sufficient hydroxyl ion from the water to lower the pH thereof, and copper and/or silver to act as an algicide and potential pH affecting compound.