The widespread presence of nanomaterials in aquatic ecosystems necessitates a careful balance between harnessing their benefits for fisheries and mitigating the associated environmental and health risks, demanding robust research efforts to protect both aquatic life and human health. Accordingly, the spherical NiO nanoparticles were synthesized using a cost- and time-efficient solvothermal strategy, and the as-formulated nanoparticles were administered to Danio rerio fish at varying concentrations. Then, the impacts of the as-formulated NiO nanoparticles on Danio rerio are extensively studied to understand their physiological and behavioral effects. NiO nanoparticles augment weight gain, specific growth rate, and relative percentage of survival (RPS) in fish by optimizing nutrient absorption and metabolic efficiency, thereby enhancing overall health and growth. Furthermore, the levels of superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPx) in Danio rerio fish are elevated by triggering an adaptive response to oxidative stress, thus bolstering the fish's antioxidant defenses to mitigate oxidative damage. At lower concentrations, NiO nanoparticles increase feeding time, foraging time, and surface time in Danio rerio fish due to mild stress-induced metabolic rate elevation and exploratory behavior, whereas at higher concentrations, these nanoparticles decrease these activities by causing toxic effects. NiO nanoparticles decrease aggression time, swimming time, and sexual behavior in Danio rerio fish due to their toxic effects, which impair neurological and physiological functions, leading to diminished activity levels and disrupted behavioural patterns as the fish experience increased stress and overall physiological decline. Thus, these findings provide a comprehensive understanding of NiO nanoparticles' influence on fish health and behavior, offering valuable insights into their potential ecological impact and directing safer practices for nanomaterial use in aquatic environments.