In what type of water is copper most toxic to fish?

Copper is most toxic to fish in soft water due to its lower concentrations of calcium and magnesium ions, which are essential for fish health. In soft water, the lower ionic concentration means that fish have a reduced ability to regulate their internal environment, making them more susceptible to metal toxicity, including that of copper.

Copper binds more readily to gills and internal physiological systems in soft water, which can lead to increased absorption of the metal into the fish’s bloodstream. This heightened sensitivity can cause various issues, including respiratory distress, reduced growth, and even mortality in fish that are exposed to copper at low levels in a soft water environment.

In contrast, in hard water, the higher levels of calcium and magnesium can mitigate the harmful effects of copper, making it less toxic to aquatic life. Brackish water may also demonstrate varying levels of copper toxicity based on salinity, while freshwater typically has a broader range of ionic composition, which further affects the interactions between copper and fish physiology.