The researchers studied how different levels of salt in water affect the toxicity of copper sulfate on young Nile tilapia fish. They found that the fish couldn't survive in water with high salt levels, but did well in lower salt levels. The toxicity of copper sulfate increased when salt levels were higher or lower, except at a moderate salt level where toxicity was lower. This study helps set safe limits for releasing heavy metals into water bodies and shows the benefits of breeding Nile tilapia in slightly salty water instead of freshwater.