Potassium tests are not very accurate. Haven't tried saliferts though.
If you have a potassium deficiency best indicators would be your caps. They will gray out and stop growing.
I had potassium issues when I used Red Sea coral pro. My green and purple caps had slow growth and a gray tinge in color.
Since I went back to ESV I no longer have issues.
I tested with salifert and red seas kits and the red sea came closest to levels in a new mixture of reef crystals. I check my potassium ever week to every other week
Potassium is extremely important for SPS Reefs. I consider it another trace element. Potassium is responsible for the vivid coloration of SPS corals.
Hmmm... I have noticed 2 monti cap frags greying out in areas and dying slowly. It's not regular rtn or stn and was curious what could be causing it. Just bought a test kit online to make sure. Thanks guys.