Disclosed herein is a method for controlling water molds in aquaculture water by using chlorine dioxide that is less toxic and safer than bronopol.The method includes adding chlorite to aquaculture water with a pH of 5.5 or higher but 8.5 or lower at a concentration of 2.5 ppm or higher but 200 ppm or lower in terms of effective chlorine dioxide and performing a reaction for 60 minutes or longer to control water molds. At this time, an organic or inorganic acid is not added to the aquaculture water. The aquaculture water includes water for hatching or water for fanning. Further, the aquaculture water includes both seawater and freshwater.
