The Biotronic® Top3 (OCAP: organic acids, cinnamaldehyde, and permeabilising complex) is used for improving microbial community in monogastric animals. This research was aimed to test the five dosages of OCAP (0%: control, 0.05%: OCAP0.05, 0.10%: OCAP0.1, 0.15%: OCAP0.15%, and 0.20%: OCAP0.2) in barramundi (Lates calcarifer) (8.3 ± 0.3 g) for eight weeks. The results showed a linear relation between OCAP level and growth rate, feed conversion ratio, protease, lipase, lysozyme in serum and mucus, alternative complement activity (ACH50) in serum, catalase, glutathione peroxidase, and malondialdehyde levels (P < 0.05). Accordingly, the OCAP0.2 treatment had the best performance in the abovementioned parameters. While there was no significant difference in expression of heat shock proteins (HSP70, HSP90) and tumour necrosis factor (TNF-a) among groups, interleukins (IL-1β, IL-8) were significantly lower in the OCAP0.15 and OCAP0.2 group than others. The expression of IGF-1 followed the growth rate trend. Generally, feeding fish with the OCAP0.2 diet improved growth rate, digestive enzymes activities, immune response, antioxidant system, and gene expressions in barramundi. As the linear relation was observed, supplementing more levels of OCAP to reach a plateau is recommended.