Sodium and Potassium metabisulfite are just different salts of the same metabisulfite moiety. They can been used interchangeably as far as I can determine in examining their Material Data Sheets. Once they are dissolved in water they dissociate into the metabisulfite and free sodium ion or free potassium ion. It is the metabisulfite part that is corrosive to metal and damaging to rubber seals and o-rings, so limit the exposure of your system (i.e. don't leave it "pickled" in this stuff for weeks or months at a time like I was advised to do early on, 2001). Use it in the recommended doses (google the data sheets if it isn't specified on the labeling) and once you have sanitized your system thoroughly rinse the system with desalinated water from your tanks. Of course you should be monitoring the TDS (total dissolved solids) and/or EC (electrical conductivity) of your desalinated water to assure that you are producing good quality water.
Best to you,
s/v Liahona SM #335