Present study was designed to quantify the dietary potassium (K) requirement of mori, Cirrhinus mrigala (initial body weight: 5.71 ± 0.02 g) juveniles. Seven isocaloric and isonitrogenous practical diets were formulated by supplementing the graded levels of K at 0, 2, 4, 6, 8, 10 and 12 g/kg, whereas the actual K level was calculated as 2.96, 5.08, 7.03, 9.07, 11.01, 13.13, and 15.09 g/kg, respectively. The feeding trial was carried out in triplicates for 60 days and fish was fed twice a day to apparent satiation. Weight gain%, specific growth rate, feed conversion ratio and organ indices were found to be best (P<0.05) in the fish fed with 9.07 g/kg dietary K supplementation. Gill Na+/K+ ATPase activity also peaked at 9.07 g/kg K. Similarly, deposition of potassium in whole-body, scales and vertebrae significantly (P<0.05) increased with the increase in dietary K supplementation up to 9.07 g/kg. The activities of liver catalase, peroxidase, superoxide dismutase and alkaline phosphatase increased (P<0.05) with an increase in K supplementation from 2.96 to 9.07 g/kg; however, further increase in K supplementation did not necessarily improve these parameters. Contrarily, thiobarbituric acid reactive substances level was observed highest (P<0.05) in fish fed with a basal diet. The data of WG% and gill Na+/K+ ATPase activity were subjected to broken-line regression analysis, and the optimal value of K was estimated to be 7.13 g/kg and 7.87 g/kg, respectively. Therefore, the inclusion of 7.13-7.87 g/kg of K is recommended in the feed of Cirrhinus mrigala juveniles.
