<p>Aquatic organisms are always exposed to environmental stress agents that provoke oxidative stress and therefore need efficient antioxidant defense systems. In the current study, cerium oxide nanoparticles (CeO₂ NPs) and iron oxide nanoparticles (Fe₃O₄ NPs) were synthesized in a green process through the application of Jatropha plant extract, and their efficacy to mimic antioxidant enzymes and remove ammonia from water was evaluated. In vitro characterization confirmed effective synthesis of CeO₂ NPs and Fe₃O₄ NPs nanozymes with catalase (CAT) and superoxide dismutase (SOD)–like activities and effective ammonia elimination by adsorption and oxidation. Subsequently, an in vivo test was conducted on common carp (<i>Cyprinus carpio</i>) to evaluate protective effects against potassium dichromate–induced oxidative stress. Fish were divided into six groups, including controls and those supplemented with CeO₂ NPs or Fe₃O₄ NPs at various concentrations. The SEM result indicated that CeO₂ NPs size is about 14&#xa0;nm, while the Fe₃O₄ NPs size is about 20&#xa0;nm. The result showed that Fe₃O₄ NPs were most effective at 5&#xa0;mg L⁻¹, whereas CeO₂ NPs had peak CAT-mimicking activity at 25&#xa0;mg L⁻¹. Fe₃O₄ NPs at 100&#xa0;mg L⁻¹ reduced ammonia from water by 43% in ammonia removal tests. Moreover, both nanozymes inhibited oxidative damage in kidney tissues, and Fe₃O₄ NPs had superior protective effects on gill tissues compared to CeO₂ NPs. These findings reveal the potential application of green-synthesized CeO₂ NPs and Fe₃O₄ NPs as multifunctional nanozymes for enhancing antioxidant defenses and aquaculture system water quality.</p>