How does elevated pH affect ammonia toxicity in fish?

Elevated pH levels significantly influence the forms of ammonia present in water, which in turn affects its toxicity to fish. In freshwater environments, ammonia exists primarily in two forms: unionized ammonia (NH3) and ionized ammonium (NH4+). The unionized form (NH3) is the more toxic of the two.

As pH increases, particularly when pH levels exceed 7.0, the proportion of ammonia that is in the unionized form increases. This shift occurs because at higher pH levels, there is a decrease in the availability of hydrogen ions, which favor the formation of the non-toxic ionized ammonium form (NH4+). Consequently, at elevated pH, fish are exposed to greater levels of unionized ammonia, thereby increasing its overall toxicity.

This relationship underscores why understanding and monitoring pH is critical in aquaculture and fish health management, as higher pH can lead to more severe physiological effects and toxicity in aquatic organisms.