The clown knifefish (Chitala ornata) is a facultative air breather that is cultured in freshwater farms in Mekong Delta, Vietnam. We report the effects of different temperatures (27 °C, 30 °C, and 33 °C) and nitrite exposure (1 mM) at these temperatures on haemoglobin derivatives, plasma ions, and acid-base status over 14 days. We also tested the effects of nitrite exposure at different temperatures on growth over 90 days of culture. An increase in temperature alone did not affect Na+, Cl−, HCO3−, and osmolality concentrations, whereas K+ was temporarily increased. Acid-base status was significantly affected by an increased partial pressure of carbon dioxide in the blood (PCO2) and reduction of extracellular pH (pHe) during elevated temperatures. Plasma nitrite levels and methaemoglobin (metHb) percentage reached the highest values at 33 °C compared to those at 27 °C and 30 °C with nitrite exposure after 3 days, but they decreased significantly at experimental termination by the detoxification process converting nitrite to nitrate. Plasma NO3− and the sum of [NO2−] and [NO3−] continuously increased during nitrite exposure at all three temperatures, reaching the highest values at 33 °C. Weight gain and survival rate increased at elevated temperatures with the highest value at 33 °C (29 g and 85%, respectively), while feed conversion ratio (FCR) values were unchanged at all temperatures. In contrast, elevated temperature caused a significant lowering of survival rate in nitrite exposure, but weight gain was highest with nitrite exposure at 30 °C and 33 °C.