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Common carp (Cyprinus carpio) is widely reared in the polyculture system in Iran. In recent years, this species has reared in concrete ponds under high culture densities. Various aquaculture practices such as high stocking densities and crowding conditions can stimulate stress responses in cultured fish (Adineh et al., 2019; Xie et al., 2008). The stress response can elevate reactive oxygen species (ROS) and cause oxidative stress, immunosuppression and susceptibility to infectious diseases (Naderi et al., 2019; Tort, 2011). Also, aquaculture is under pressure to decrease the application of antibiotics due to the spread of antibiotic-resistant pathogens and food security problems (Chakraborty & Hancz, 2011). Under these conditions, herbal products have been used as immunostimulants in aquaculture to avoid the careless use of dangerous antibiotics. Herbal products can be used in fish farming as alternatives to vaccines, chemicals and antibiotics to prevent and control diseases in fish (Citarasu, 2010; Galina et al., 2009). Thus, the potential use of herbal products as immunostimulants in aquaculture is a promising novel strategy and further studies are beneficial. Many herbal compounds as natural antioxidants inhibit the production of ROS and scavenge free radicals (Chakraborty &

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This study is aimed at evaluating the effects of stocking densities and dietary protein levels in the biofloc system on the performance of common carp. Fish ( 12.09 ± 0.99  g) were transferred to 15 tanks: fish reared at 10 kg/m3 as medium density and fed 35% (MD35) or 25% (MD25) protein and fish reared at 20 kg/m3 as high density and fed 35% (HD35) or 25% (HD25) protein in the biofloc system and control fish reared at MD and fed 35% protein in clear water. After 60 days, fish were subjected to crowding stress (80 kg/m3) for 24 h. The growth of fish was highest in MD35. The feed conversion ratio was lower in MD35 compared to the control and HD groups. The amylase, lipase, protease, and superoxide dismutase and glutathione peroxidase activities in the biofloc groups were significantly higher than in the control. After crowding stress, cortisol and glucose levels in biofloc treatments were significantly decreased compared to the control. After 12 and 24 h stress, lysozyme activity in MD35 was significantly lower than in the HD treatments. Overall, the biofloc system with MD could improve growth and robustness against acute stress in fish. Also, biofloc could compensate 10% reduction of protein in common carp juvenile diet when reared in MD.