Copper is the most toxic trace elements in plants. Very low concentration of it can seriously poison fish and plants, especially while treatment of fish diseases or removing algae. Adding excessive copper sulfate cause copper poisoning. The water hardness can affect copper toxicity: for instance, under 1gH, the water containing 0.1ppm copper will lead to fish or plants intoxication; under 15gH, only 0.2ppm of copper will lead to toxicity.