The present study investigated the effects of water pH (5.0, 7.0 and 9.0), hardness (0, 20 and 120 mg CaCO3 L−1) and temperature (15, 23 and 30 °C) on the induction of sedation and anaesthesia, and subsequent recovery, of silver catfish exposed to eugenol. Moreover, the blood gas tensions (PvO2 and PvCO2) and blood pH in silver catfish acclimated to these temperatures were investigated after exposure to eugenol. Water pH, hardness, temperature and fish size affect the efficacy of eugenol in silver catfish, particularly at the lower concentrations tested (20 and 30 mg L−1). Sedation of this species can be induced at concentrations as low as 20 mg L−1, but for anaesthesia, a concentration of at least 40 mg L−1 of eugenol must be used to compensate for the influence of fish size and water quality. Blood gas tension and pH were affected by eugenol anaesthesia, but only in fish acclimated to 30 °C.