The optimal level of Cosmos caudatus supplementation in fish formulation was conducted by using Mystus nemurus fingerlings in two initial sizes (3.05±0.39 g and 4.45±0.71 g). For Experiment 1, crude leaves incorporated were 0.00, 0.50, 1.50, and 2.00% with 60 days of feeding trial. For Experiment 2, the level of leave extract was 0.00, 0.01, 0.05, 0.10 and 0.20% with an experimental duration of 90 days. Both experiments showed significant differences (P<0.05) for growth performance and condition factor. The highest weight gain for Experiments 1 and 2 were 13.28±3.07 g and 18.51±4.21 g, respectively. There were no significant differences in the whole body content and survival rate for both experiments (P>0.05). The protein level was in the range of 15.70±0.79 to 16.74±1.90% (Experiment 1) and 14.96±1.90 to 15.67±0.55% (Experiment 2). The optimal level of crude leaves and extracts was considered at 0.50% and 0.05%, respectively, for M. nemurus fingerlings.
