Hydrogen Sulphide is a gas characterized by a distinct rotten egg odor. It often occurs in conjunction with iron and manganese. It is very corrosive to plumbing, kills aquarium fish, tarnishes silver, and at high level is dangerous to health. It can be tested at levels as low as 0.5 ppm. When present it must be removed before the water is suitable for human use. This is usually accomplished with reverse osmosis, activated carbon filtration, green sand filtration, aeration, or ozonization.