A 90‐day feeding trial was carried out to investigate the effect of varying dietary protein on growth performance, health status, intestinal enzyme activity, carcass composition and intestinal histomorphology of common carp (Cyprinus carpio) fingerlings for defining the optimum dietary protein. Five groups were fed with different crude protein content: T1 (36%), T2 (38%), T3 (40%), T4 (42%) and T5 (44%). Each group was run in triplicate with 10 fingerlings per tank. The group fed with 38% crude protein showed significantly (p < 0.05) higher specific growth rate, daily weight gain, weight gain percentage and low feed conversion ratio as compared to groups which had a protein content lesser/higher than 38%. The body composition and digestive enzyme activity were significantly higher in T2 followed by T1 (36%). The excretion of total ammonia and orthophosphate was also lowest in T2. The intestinal histomorphology examination revealed significantly increased absorptive area in T2 compared to other treatment groups. The findings indicate that optimum amount of dietary protein is crucial for achieving maximum growth, and feeding fish with diet containing excess or lesser than optimum protein content leads to lesser growth besides more excretion thereby affecting the quality of water.