This study aimed to assess whether the addition of calcium carbonate (CaCO3) to increase Calcium levels reduces the long-term nitrite (NO2−) toxicity in pacu Piaractus mesopotamicus juveniles. The fish were exposed for 60 days to six treatments comprised by the interaction of three NO2− concentrations (0, 15 and 30 NO2−-N mg L−1) and two Calcium levels (60 and 150 CaCO3 mg L−1), all in triplicate. The zootechnical indices, as well as blood and oxidative stress parameters were evaluated at the end of experimental period. The results showed that at lower hardness, NO2− exposure led to enhanced lipid peroxidation (LPO) levels of muscle and gills and compromised the food conversion rate. Otherwise, the LPO levels were kept at basal condition and no changes were found in zootechnical parameters at higher hardness treatments. Thus, the increase in Calcium levels up to 150 CaCO3 mg L−1 using CaCO3 may reduce the NO2− toxic levels in pacu juveniles, and may be an interesting strategy of water quality management, especially in intensive production systems. •The prolonged exposure to nitrite (NO2-) levels result in tertiary stress responses in pacu Piaractus mesopotamicus.•The addition of calcium carbonate (CaCO3) to increase water hardness reduces the long- term NO2- toxicity in juvenile pacu.•At lower hardness, NO2- exposure led to enhanced levels of lipid peroxidation (LPO) of muscle and gills in juvenile pacu.•The increase in hardness is an interesting strategy of water quality management, especially in intensive production systems.