Salt adds minerals to your water and acts as a buffer. The addition of salt will raise pH and hardness in your tank.
Lots of things can raise pH. Carbonate rocks are the most common culprits. Sea shells as well.
Lowering pH can take an awful lot of work, especially if you have hard water. Perhaps if you describe your situation a bit better, as in what you are trying to accomplish with your tank, I can help you a bit more.
~~Colesea