Man, our water in Pocatello was so hard, we used to throw it into rock beds to watch it break!
All kidding aside, the water was really really hard, and taking a bath was a joke! Get a water softener. Call your local plumbers and ask them to give you some suggestions on brands and solutions. Your local plumbers know the condition of your water about as well as anyone.
Carbon is definately a filter, not a softener, and this is the second time I've seen a question about potassium, but I am not yet aware of any potassium water softeners. The classic softener uses rock salt.
On the main board, use Ctrl-F and search for "water softeners 101" for an excellent article describing the process.