The relationship between water pH and hardness influences fish physiology, where increasing water hardness could trigger a protective effect for fish exposed to acidic or alkaline pH. Hence, this study aimed to investigate pacu (Piaractus mesopotamicus) for the existence of a protective effect of high water hardness at acidic and alkaline pH. Pacu exemplars were exposed to low (LWH, 50 mg CaCO₃ L⁻¹) or high water hardness (HWH, 120 mg CaCO₃ L⁻¹) at acidic (5.5), circumneutral (7.5) or alkaline (9.0) pH, for 15 days. Un-ionised ammonia was the highest at the alkaline pH, and alkalinity was the lowest at the acidic pH. Blood pH levels were lowest at alkaline pH, mainly when fish were subjected to HWH. The HWH presented a protective effect for pacu exposed to acidic or alkaline pH, as verified in the blood glucose and lactate, and plasma total ammonia levels on day 1, total plasma protein levels on day 5, and total ammonia and alkaline phosphatase levels in plasma on day 15. Values of haematocrit, haemoglobin and haematimetric indices, were typically increased in fish exposed to alkaline or acidic pH in LWH throughout the experiment, which, in turn, was not verified for fish exposed to HWH. In conclusion, pacu can cope with acidic or alkaline conditions, but acidic and alkaline pHs under LWH are stressful. The combination of acidic or alkaline pH with HWH triggered a protective effect because biochemical and haematological variables of pacu juveniles showed few changes in fish exposed to circumneutral pH.