Aniseed, Thyme, and Basil Essential Oils as Functional Feed Additives for Common Carp in Recirculating Systems

This study examined the effects of dietary aniseed, thyme, and basil essential oils (EOs) on growth, health, and tissue integrity of common carp (Cyprinus carpio) reared in a recirculating aquaculture system (RAS). Juvenile carp (102 ± 2.8 g) were fed for 12 weeks four isonitrogenous diets: a control and three diets supplemented with 0.2% aniseed (V1), thyme (V2), or basil (V3) oils. Growth performance was not significantly affected (p > 0.05). Flesh biochemical composition improved, with higher protein in V1 (17.85 ± 0.22%) and lower fat in V3 (1.78 ± 0.21%) compared to the control. Hematological parameters and antioxidant enzymes (SOD, CAT, GPx) indicated enhanced immune and oxidative status, while MDA (malondialdehyde) levels decreased. SOD activity increased in treated groups, reaching 4.329 U mg−1 protein in muscle and 4.908 U mg−1 protein in liver in V2, compared to 2.775–3.677 U mg−1 protein (muscle) and 3.508–4.349 U mg−1 protein (liver) in controls. CAT activity was highest in the same group 57.045 U mg−1 protein versus 31.403 U mg−1 protein in controls. Microbiological assessment revealed reduced total aerobic bacteria and Enterobacteriaceae counts in EO-fed fish. Histological analysis showed healthier hepatic and intestinal structures, reduced vacuolation, intact epithelium, and abundant goblet cells in EO-treated groups.