A fast acting water purification system containing a source of silver ions which is suitable for use in personal or household water containers, where the non-potable water may contain halides or other materials that limit the solubility of silver in the non-potable water, with the purification agent including a source of silver ions and a compound containing a hydantoin ring increase the presence of silver ions in the non-potable water to a level sufficient to quickly kill harmful microorganisms in the non-potable water without the need to add additional biocides to the non-potable water or pretreat the non-potable water.