Iron in your water is the most likely culprit. It stains faucets, sinks, bathtubs, countertops, appliances and even clothing. You can usually remove the iron with a home water conditioner.
In some instances, iron is present in such large amounts that a multi-stage filtration system is necessary to remove it. But don’t fret; your problem may appear worse than it really is. Iron in even small amounts can cause staining.