High density fingerling rearing of pengba, Osteobrama belangeri in biofloc system was carried out for 45 days with wheat flour and molasses as two carbon sources to maintain C/N ratio of 15:1. Effect of biofloc was evaluated in terms of water quality changes, fish growth performance, digestive enzyme activities, non‐specific immune system and antioxidants enzyme activities. Use of the two carbon sources ensured prevalence of better growing environment, higher digestive and antioxidant enzymatic activities, ultimately leading to higher fish growth, as compared to control. Further between the two carbon sources, use of wheat flour caused significant increase in the activities of digestive enzymes, amylase and total protease (p < .05); antioxidant enzymes such as SOD and catalase; increase in serum total protein, albumin and globulin, as compared to molasses, ultimately leading to higher growth. However, non‐specific immune parameters such as respiratory burst activity in blood, lysozyme activity, and myeloperoxidase activity in serum were higher (p < .05) in fish reared with molasses. Such result while indicated benefits of additional carbon source, it also revealed wheat flour to be a better source for carbon supplementation than molasses in biofloc system during the high‐density fingerling rearing of O. belangeri.