Which one of the following metals is most fatal to fish when it becomes dissolved in acid waters?

Aluminum is particularly toxic to fish when it becomes dissolved in acidic waters. This is due to the fact that lower pH levels significantly increase the solubility of aluminum, allowing it to enter the water in more bioavailable and toxic forms. When fish are exposed to high concentrations of dissolved aluminum, it can impair their gill function, disrupt osmoregulation, and negatively affect their immune systems, leading to increased mortality rates.

In acidic conditions, aluminum can also precipitate out of solution as the pH increases, which can lead to further problems if the water conditions fluctuate. The heightened toxicity of aluminum in lower pH environments makes it a significant pollutant in aquatic ecosystems, particularly in areas impacted by acid rain or acidic mining runoff. Understanding this relationship highlights the importance of maintaining pH levels in water bodies to protect aquatic life from heavy metal toxicity.