Freshwater hardness affects metal toxicity and fish physiology. Therefore, the effects of dissolved metals on fish physiology may differ in waters from different geographic regions. In this study, toxic effects of Cd and Cu (1 mg/L) on the antioxidant system of freshwater fish Oreochromis niloticus were investigated in two different waters (commercial and tap water). Fish were exposed to metals in hard water (similar to 320 mg CaCO3/L, conductivity 5.80 mS/cm) and soft water (similar to 80 mg CaCO3/L, conductivity 1.77 mS/cm) for 1, 7 and 14 days and consequently the activities of antioxidant enzymes (superoxide dismutase, SOD; catalase, CAT; glutathione peroxidase, GPX; glutathione reductase, GR and glutathione S-transferase, GST) and total glutathione (GSH) levels were measured spectrophotometrically. Cu exposure of fish in soft water caused fish mortality after 8 days, though there was no fish mortality in the other conditions. The antioxidant system of fish responded differently to metal exposures in waters with differing hardness. Metal exposures in soft water showed predominant effects on the antioxidant system of fish comparing to hard water exposures. Similarly, antioxidant enzyme activities also altered in control fish depending on the hardness of waters. In general, CAT was the most sensitive antioxidant enzyme followed by SOD and GPX. CAT and GSH showed an increasing trend while a decreasing trend was observed for SOD and GPX. This study emphasized that the water chemistry affects the fish antioxidant system and metal toxicity that may be useful in environmental monitoring and also evaluating biomarkers in fish from different regions.