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.

Abstract: An inexpensive cation exchanger useful in purifying waste water is produced by treating comminuted bark in separate steps with an alkali hydroxide solution of at least 5% by weight, preferably 20-40% by weight, and with 30-75% by weight sulphuric acid, preferably 50-65% by weight. After each step washing with water is performed until the pH-value after the alkali hydroxide treatment goes below 9 and after the sulphuric acid treatment goes above 3. Preferably, the treatment with alkali hydroxide precedes the treatment with sulphuric acid. The treatment with alkali hydroxide is carried out for 0.5-20 hours, preferably 3-10 hours, and the treatment with sulphuric acid is carried out from 0.5-8 hours, preferably from 1-6 hours. The cation exchanger can be regenerated with alkali hydroxide solution, particularly sodium hydroxide solution.