Abstract: The present invention relates to a method of purifying water for fish-breeding, according to which the water is optionally subjected to sedimentation, is subjected to sand filtration and to oxygenization, and to which the water also is subjected to a treatment with an ion exchanger mixture consisting of 40 to 70 percent clinoptilolite, 30 to 15 percent anion exchanger and 30 to 15 percent cation exchanger, the two latter being of cellulose type. The invention relates furthermore to the ion exchanger mixture mentioned.