Bummer, glad you figured it out, I keep a temp probe in my sump and one in my main tank just for that reason...So I posted a bit ago as algaefix does remove chrysos. And I also stated my sps started to STN. Well. I figured out why, and it wasn’t due to algaefix.
at first I was stumped. Did an ICP test and results came back with no metals etc. even my BTA split. That was when I knew something else was a problem. It turns out my turn over rate wasnt fast enough to keep the display water warm. And caused dramatic temp drops during this winter..