Acid and alkaline death points for fish are approximately pH?

• A. 2 and 9
• B. 4 and 11
• C. 3 and 10
• D. 5 and 12

Answer: (B)

Fish can tolerate only a limited range of water pH before their bodily systems fail. When pH becomes very low, the acid stress disrupts enzyme activity, membrane integrity, and ion balance, making it hard for the fish to regulate internal pH and maintain respiration and metabolism. When pH becomes very high, alkaline stress also disrupts enzyme function and gill ion transport, and the proportion of toxic ammonia shifts toward the uncharged form (NH3), which increases toxicity and stresses the respiratory system. Across many freshwater species, the approximate extreme points at which death occurs are around pH 4 on the acidic side and pH 11 on the alkaline side. This makes the 4 and 11 pair the best match. Other options propose thresholds that are either far more extreme or mistimed for typical fish tolerances, such as pH 2 (far too acidic for most fish), 9 (often tolerable), or 12 (much more extreme than usually lethal for general cases).