Heavy metals like zinc -- which are present in industrial effluents when they are released into water bodies -- can accumulate in fishes, causing toxicity. Korean scientists studied the changes in goldfish who were exposed to different concentrations of zinc and water hardness. They found that increasing the hardness (i.e., the level of certain positively charged ions) in water can inhibit the absorption of zinc and reduce heavy metal-induced toxic stress and degeneration in these fish.
