To maintain an outdoor cold plunge tub or ice bath with no filtration system, you should use approved chemical sanitation methods as well as drain and clean it regularly to prevent the buildup of bacteria, algae, and other contaminants. For cold plunge tubs that include sanitation and filtration systems, consult the manufacturer for specific maintenance requirements, but these generally include regular changing of the filter and periodic replacement of tub water—depending on frequency of use. It's also important to use a skimmer to remove any debris that crept into the tub from the outdoors, and cover the tub when not in use to prevent debris from entering.