Muriatic acid is Hydrochloric acid and is available at most hardware stores and home improvement places. Be very careful handling this stuff. Wear eye protection and gloves. Always add acid to water and not water to acid otherwise you can get a violent boil. The other key thing in pool care is to go slow and easy...patience. Add a little of something, circulate for a few hours, then test. Do this until you have 'calibrated' your pool and you know from experience and measurements exactly what and how much of something to add for the results you want. It will take some time but you'll get the hang of it. Enjoy your pool.
Al