Hi there, welcome to AA!
What kind of salt mix are you using? I would reconfirm the reading to be sure as 15 is awfully high. It is possible you have extremely hard tap water, and it is throwing off the reading. The way to lower it would be to use RODI
water to mix your saltwater. At readings that high, you could end up having issues with Ca
and coral growth (if you have coral) as high alk
like that can cause precipitation of CaCO3 before coral can uptake the ions in suspension.