Interesting site. A bit expensive but laboratory grade measurements would be wonderful to have.
The site had his to say about Potassium:
In natural seawater, potassium is a non-conservative major element with a concentration slightly lower than that of calcium. It is a component of aragonite, and regular dosing has, within the past several years, has been implicated in improving the blue coloration of numerous varieties of small-polyp stony corals; the benefits of potassium supplementation are potentially two-fold, then: provision of an element that is 1.) incorporated into the skeletal material of corals and other reef-building organisms for purposes of growth, and 2.) incorporated into pigments that enhance blue coloration of small-polyp stony corals. The importance of potassium to marine organisms is most apparent when beginning to dose it in aquaria with depleted potassium concentrations and/or in which the sea salt mixture used is potassium-deficient; in such cases, changes in the appearance of many corals may be observed within the first weeks of regular dosing.