The effects of dietary Nano selenium (Se) and garlic extract on the performance of grass carp under different stocking densities were examined. Fish (2.10 ± 0.09 g) were stocked into 18 tanks: fish were fed a diet supplemented with 1 mg Nano Se and 1 g garlic kg−1 diet (Diet1) or a diet supplemented with 2 mg Nano Se and 2 g garlic kg−1 diet (Diet2) and reared at low (LD, 24 fish per tank), medium (MD, 48 fish per tank), and high (HD, 96 fish per tank) stocking densities. After 60 days, growth was highest in Diet2‐LD and Diet2‐MD groups and lowest in Diet1‐HD group. Results showed an extreme low or high density induced stress responses. At all densities, serum cortisol and glucose levels were significantly lower and amylase and lipase activities were higher in fish fed diet 2. The protease, catalase, and GPx activities were highest in the Diet2‐MD group. The highest SOD activity was observed in medium‐density groups. The serum malondialdehyde level was lowest in Diet2‐LD and Diet2‐MD groups. Thus, higher dietary levels of Nano Se and garlic are recommended to suppress stress and improve growth, digestive activity, and antioxidant capacity in grass carp. In addition, farming fish at medium density is the most optimal aquaculture practice.