I have been using the Potassium supplement from KZ for some time now. I started using it due to fading colors in my SPS. All salt mixes except SeaChem Reef salt are considerably lower in K than NSW. I theorize in a low nutrient environment and with the addition of carbon sources for bacterial processes, the various trace elements, especially K become more available to be skimmed out, causing a rapid depletion of this element. I use 3ml daily in 340 total gallons. Marc is correct in that there is no commercially available test kit for K. I'm not familiar with Potassium Chloride, so I guess I didn't answer your question directly.
CAReefer