Method for reducing harmful organisms on surfaces in contact with continuously or periodically flowing currents of cooling water used for throughflow cooling without recirculation, wherein the harmful organisms are algae, mussels and slime forming organisms, by periodically adding an aqueous treatment agent containing at least one peroxycarboxylic acid with 1 to 6 C atoms to the flowing water once every 5 to 100 hours over a period of 3 to 60 minutes in an amount corresponding to at least 1 ppm of peroxycarboxylic acid to reduce growth of the organisms on the surfaces.