Objective: The objective of the present study is to study the toxicity of Magnesium oxide nanoparticle and Magnesium oxide bulk against two fishes (Tilapia (Oreochromis mossambicus) and Zebra fish (Danio rerio) for toxicity. The Biochemical and antioxidant activity were also studied in both control and the injected fish. Bioaccumulation was also studied. Methods: Magnesium oxide bulk and Nano particles were prepared by dissolving it in 1μg/mL concentration and sonicated for half an hour. Different concentrations of Magnesium oxide both bulk and nano particles from 10 to 200 ppm were determined. Results: There was a significant decrease in protein. In the Catalase activity there was a gradual and sporadic increase. The specific activity of GST enzyme increased significantly with an increase in the concentration of Magnesium nanoparticles. There was an increase in the concentration of Magnesium nanoparticles and more accumulation was seen. Conclusion: Heavy metals are common pollutants and restricted to frequent inhalation in aquatic organism due to their larger size. The results revealed that magnesium oxide bulk particle was found to be more toxic when compared to nano particle.