Couple of additional questions. How long has your tank been up and running? Is this a wild colony? What are your temperature swings like from night to day? Do you know what your ph is? There is a direct correlation between Alk, CA, ph and Mg. I would personally use Elos or Salifert test kits. I have never had luck with API, maybe its just me. Also are you using an Auto Top Off to keep the salinity stable. In my experience Milles are sometimes the first to show signs of stress. I would do another check for redbugs and AEFW.