The effect of mercury (Hg) toxicity, its impact on liver histopathology, hematological and biochemical changes in Nile tilapia (Oreochromius niloticus) were studied. The amelioration effects of Spirulina platensis and Chitosan were investigated through a semi-static acute toxicity test developed with mercury chloride (HgCl2). Fingerlings (4.45 ± 0.31 cm and 2.35 ± 0.18g) were kept during 96 hours in 5-liter glass aquaria, according to the following mercury concentrations, set up in three replicates: 0.00 (control 0.05, 0.10, 0.20, 0.30, and 0.40 mg Hg L1. The 96 h LC50 value for Hg in O. niloticus fingerlings was calculated by the simple graphic method to be 0.30 mg HgCl2 L-1. Fish exposed to Hg resulted in significant reduction (P< 0.05) of the erythrocyte count (RBCs), hemoglobin content (Hb) and hematocrit value (Hct). Significant changes in plasma aspartate aminotranseferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP) and acid phosphatase (ACP) were observed in fingerlings exposed to Hg. Results also, indicated that the addition of Spirulina platensis or chitosan to the Hg polluted media reduced significantly (P<0.05) Hg level in aquarium’s water as compared to that of Hg alone. They improve the hematological parameters (RBCs, Hb, Hct) and ameliorate the toxic effect of Hg in the aquatic environment.