Nickel (Ni) as a waterborne metal pollutant is widespread and harms fish health. In this study, the effects of fortifying diets with 0.1% Melaleuca alternifolia (MO) and 0.1% Ocimum basilicum (BO) essential oils on behaviors, neurotransmitters, liver and kidney functions, biochemical indices, and tissue histological features were studied in Nile tilapia (Oreochromis niloticus) under Ni exposure conditions. A total of 240 fish (27.92 ± 0.22 g) were equally classified into six groups, each with four replicates of 10 fish. The control, MO, and BO groups were fed basal control, 0.1% MO, and 0.1% BO diets, respectively, and reared in Ni-free water. The Ni, Ni + MO, and Ni + BO groups were reared in Ni-medium at a concentration of 3.6 mg/L and fed the same feeding regime as the control, MO, and BO groups, respectively. Ni exposure induced behavioral alterations, including lowered foraging (coefficients of variation, CV = 2.73-14.29%), swimming (CV = 2.95-19.23%), mouth pushing (CV = 9.12-29.37%), spreading of the tail (CV = 3.13-17.32%), and approach (CV = 3.53-11.27%) patterns but increased surfacing (CV = 11.39-23.33%) and resting (CV = 7.14-25%) behaviors. Increased mortality (CV >
30%) and hepato-renal indicators were consequences of Ni exposure. The Ni exposure also depressed brain acetylcholine esterase (AChE) (CV = 0.54-1.10), blood total protein (CV = 3.23-21.87%), albumin (CV = 1.62-13.47%), and globulin (CV = 1.91-24.72%). Histopathological changes were observed in the brain, gills, liver, kidney, and muscle, with detectable Ni residues in the muscle of Nile tilapia. Dietary supplementation with 0.1% MO and/or 0.1% BO significantly improved behavior patterns, blood proteins, and AChE levels under Ni exposure conditions. Hepato-renal indicators and the histology of the studied organs were enhanced, and the residual Ni level was reduced by feeding on the tested diets. Based on the current results, it was concluded that essential oils (0.1% BO and/or 0.1% MO) diets could mitigate the harmful effects caused by Ni exposure in Nile tilapia, which might enhance their future application as effective feed additive candidates in aquaculture.