A water purification system for a swimming pool, pond, aquatic mammal tank, spa or fountain, which is simple to operate, low in maintenance, and highly reliable. The system takes advantage of synergistic benefits derived from the combination of ozonation and electrolytic chlorination in the same system. Combining both of the technologies of Ozonation and Electrolysis allows as in-site production of at least one of chlorine dioxide that is used as a disinfectant agent for treating water reservoirs.