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 comprises at least an ozonator and an electrolytic chlorinator. The system preferably comprises, in sequence, a vortex separator for initial separation of particulates; optionally an absorption or adsorption filter which may be a conventional pool filter using sand, diatomaceous earth, or paper cartridge as filtering medium; an ozone injector; an ozone contact chamber having a top end and a bottom end, wherein said water stream injected with ozone enters at or near said top end and is withdrawn from at or near said bottom end; and an electrolytic chlorinator for production of chlorine which is then oxidized to oxychlorine species.