Abstract: Fish were exposed to either soft water or hard water containing 1,4,8 and 12 ppm ZnSO4.The sublethal effects of Zn2+ on respiration and cough frequencies were studied over a period of 15 days.Changes in the respiratory activities were recorded after the addition of EDTA.Methods for detoxication were explored.The results are as follows.(1)Respiratory frequency and cough response were apparently affected by Zn2+ concentration.The higher the Zn2+ concentration,the further the respiratory frequency curve deviated from the control,and the higher the cough frequency.These responses can be used as indices for the physiological monitoring of toxication and detoxication in fish.(2)Zn2+ toxicity was apparently lower in hard water than in soft water.(3)EDTA can eliminate or lessen the toxic effect of Zn2+;its detoxication effects were similar in soft and hard water.