You need salt that is at least 99% pure, with no additives or anti-caking agents. You can usually find this kind of salt at Home Depot or at a pool store. You can aslo use rock salt for water softeners, but it will take much longer to dissolve. If you want to increase the level from 0PPM to say 3000PPM, then you would need to add around 560lbs, or 254kg of salt.