Here's my blurb about potassium from an upcoming R2R article:
Potassium is listed with the less critical parameters, not because it isn't important, but due to the fact that it does not get rapidly depleted in most aquaria. The majority of reef aquarists do not test for or dose potassium, and likely have adequate amounts from water changes alone. Potassium is important for cellular function, and generally is higher in concentration inside of cells than outside. In people, for example, nearly all of it is inside of cells, with very low concentrations present in the blood.
In marine systems, most cells of organisms have higher concentration of potassium in them than the surrounding seawater. That would make it seem that potassium would be depleted rapidly as organisms grow and add tissue mass, whether they are bacteria, microalgae, macroalgae, fish, or corals. However, there is quite a lot of potassium in seawater and salt mixes, and aquaria are typically being feed foods that also consist largely of cells that once contained potassium. Assuming these cells are not broken open and rinsed free of potassium, a large amount comes in with foods. So the net concentration of potassium in the tank will be a balance between the food and other inputs, and the uptake from tissue mass (whether it is exported or left in the tank).
A number of aquarists have found their aquaria are depleted in potassium and dose it to maintain natural levels. I've not found it to be depleted in my aquarium and I do not dose any. Some people associate depletion with organic carbon dosing to drive bacterial growth, but I've not seen that in my system (perhaps due to the foods that I choose to feed). Of those with depleted potassium, the primary symptom seems to be certain issues with SPS corals such as Montipora. Sometimes it is reported as poor growth and/or greyish coloration. I do not know if that really does relate to low potassium, but if you have such an issue, measuring potassium with a kit and dosing if necessary may be useful. I'd recommend maintaining about 380-420 ppm, but if it already is higher than that level, I would not do anything to try to lower it.
