Residue and stains in the bathtub are often caused by low pH and acidic water. These conditions result from high amounts of carbon dioxide in the water supply. The acidity can lead to the corrosion of plumbing materials, which in turn causes rusty, blue, or green stains to appear in your bathtub. These stains not only affect the appearance of your bathroom but can also indicate underlying issues with your water quality that may need to be addressed.