Nitrite and carbon dioxide are common environmental contaminants in the intensive aquaculture ponds used to farm clown knifefish (Chitala ornata) in the Mekong delta, Vietnam. Here we tested the hypothesis that hypercapnia reduces nitrite uptake across the gills, because pH regulation will reduce chloride uptake and hence nitrite uptake as the two ions compete for the same transport route via the branchial HCO⁠3⁠−/Cl⁠− exchanger. Fish fitted with arterial catheters were exposed to normocapnic/normoxic water (control), nitrite (1mM), hypercapnia (21mmHg CO⁠2), or combined hypercapnia (acclimated hypercapnia) and nitrite for 96h. Blood was sampled to measure acid-base status, haemoglobin derivatives and plasma ions. Plasma nitrite increased for 48h, but levels stayed below the exposure concentration, and subsequently decreased as a result of nitrite detoxification to nitrate. The total uptake of nitrite (evaluated as [NO⁠2⁠−]+[NO⁠3⁠−]) was significantly decreased in hypercapnia, in accordance with the hypothesis. Methemoglobin and nitrosylhemoglobin levels were similarly lower during hypercapnic compared to normocapnic nitrite exposure. The respiratory acidosis induced by hypercapnia was half-compensated by bicarbonate accumulation in 96h, which was mainly chloride-mediated (i.e. reduced Cl⁠−influx via the branchial HCO⁠3⁠−/Cl⁠− exchanger). Plasma osmolality and main ions (Na⁠+, Cl⁠−) were significantly decreased by hypercapnia and by nitrite exposure, consistent with inhibition of active transport. We conclude that hypercapnia induces a long-lasting, and mainly chloride-mediated acid-base regulation that reduces the uptake of nitrite across the gills.