I don’t really have an overwhelmingly big hard water problem. My white laundry doesn’t turn rust color, and the other toilets don’t exhibit a major problem, but one of the 3 builds up huge mineral deposits. It’s the same exact toilet as one of the other toilets, and like I said, all connected to the same water from the city. I would have more expected either all of them or none of them to have a problem, but just one?
Very old house, but replumbed, so it’s all PVC pipes, circa about 2000.