Sodium balance in the catfish Clarias mossambicus exposed to acid water

African catfish, Clarias mossambicus showed no ill effects when kept in acid water pH 4–5 for 3–4 days. Their ionic regulatory response was examined and during 24 h exposure to water of pH 4, Na⁺ efflux increased significantly but was not matched by an increase in Na⁺ influx resulting in a substantial net loss of body Na⁺, which was reduced from 65.4 mmol · kg^?1 to 35.6 mmol · kg^?1, with smaller losses of Cl^? and K⁺.