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Biofloc technology offers a viable choice for the rearing of fish seed by offering a safe and protected habitat for young fish. Nevertheless, it is crucial to establish a standardised stocking density and implement effective ameliorative steps in order to successfully utilise this technology. In this study, a 90‐day investigation was conducted to assess the effect of stocking density and dietary vitamin C (Vit C) levels on the growth and immunity of Labeo gonius fry (1.03 ± 0.01 g) reared in a biofloc system. Three stocking density groups (5, 10 and 15 fish per 50 L) were established, and each experimental group was supplemented with three levels of Vit C (0, 500 and 1000 mg kg−1). The highest survival rate was observed in the lowest density group (five fish per 50 L) fed with 1000 mg kg−1 Vit C. A better feed conversion ratio and significantly higher protein efficiency ratio were found in the moderate stocking density group (10 fish per 50 L) fed with 500 mg kg−1 Vit C. Total leukocyte count (TLC), haemoglobin, and packed cell volume improved in Vit C‐fed groups. The total erythrocyte count (TEC) increased in groups fed Vit C and stocked at lower densities (5 and 10 fish per 50 L). Total serum protein (TPP) content increased when Vit C was added at a rate of 500 mg kg−1. Serum glucose and cortisol levels were significantly reduced in Vit C‐supplemented groups. Supplementation of Vit C at 500 mg kg−1 resulted in a significantly lower value of malondialdehyde (MDA). Thus, the findings confirm that the incorporation of Vit C in the basal diet promotes the growth and health status of L. gonius fry reared in the biofloc system at high‐density rearing. [ABSTRACT FROM AUTHOR]