I'm a newbie myself, however according to the "Beginner's Guide to Pool Chemistry Needs" sticky posting on this site (at http://www.poolforum.com/pf2/showthread.php?t=1113), Muriatic Acid LOWERS Ph. It does not raise it. According to that posting, Borax or Soda Ash raises it.
So, if he did tell you to use Muriatic Acid, maybe he's a newbie as well and you may want to watch out.
I'd let an expert confirm all this though (because as I said, I'm not one). Just wanted to post this before you added it though.
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