It depends. If you mix up RO or RO/DI water with your salt, aerate it for about a day, and get low pH and/or alk readings, you can use products like Kent Marine's 'Superbuffer dKH' or Seachem's 'Reef Buffer'. Both products will increase both pH and alk. Of course, that just two of many products out there...lots of different ways to go about this.
I used the Kent product for a while...until I bought a Salifert test kit and figured out that my old one (Hagen) was giving me false low alk results. After mixing and aerating, my salt water comes out with KH at about 10 and a pH of 8.1