Use of at least two polymers of quaternary ammonium compounds and its salts, for disinfection and algae prevention in bathing water, is claimed. At least one polymer has a polymerization degree of 30-55 and at least one other polymer has a polymerization degree of 120-160. An independent claim is also included for a medium for disinfection and algae prevention in bathing water comprising at least two polymers of quaternary ammonium compounds or its salts.