Nitrate in Water
An excess of nitrate in water may result in eutrophication. This causes an overgrowth of algae, which depletes oxygen to levels that are too low for fish.
Drinking water with a high concentration of nitrate can cause serious illness or death, as it reduces the ability of blood to carry oxygen. This is especially dangerous for infants under six months. This is why the US EPA regulates the amount of nitrate-nitrogen in public drinking water.