Muriatic acid is compatible with ALL pool treatment systems, even bogus ones like Permasalt.

BUT . . . simply adding acid will NOT lower alkalinity. You need to read the articles linked in this FAQ: http://www.poolforum.com/pf2/showthread.php/191

You should also read the Permasalt info here: http://www.poolforum.com/pf2/showthread.php/29112

Good luck!