Those looking to dose potassium should do so carefully and slowly. Overdosing potassium can be lethal to the coral. There are several kits on the market and some of them can tell you if potassium is below 400 ppm. If it is, you will generally want to correct this. It can be very hard to grow certain coral in low potassium. Most commercial salts create a mix that is near 400 ppm. Marine aquariums should be maintained at least around 400ppm. I have been running certain systems higher, 600-800+ ppm. If you can’t find a test kit that meets your needs, you can use a company where you send in your water for testing, they have higher range potassium testing capabilities.
Potassium is sometimes added to aquariums through various means. One obvious example is the salt mix itself. There are also two part additives which may contain trace amounts of potassium, general element supplements, as well as potassium iodide, the standard softie support solution. I dose a potassium chloride solution in a 5 gallon bucket and let it drip feed into the system overnight. Any dosing regimen should start very conservatively, and very slowly over time should you increase dosage.
There are several potassium aquarium products on the market. I started dosing at 2 tablespoons dry powder in approx 800 gallons, and increased over time to about ½ cup a day depending on dosing. I always mix this several gallons of RO and drip it in over the course of the day. Potassium chloride powder should not be added directly to the aquarium as this will harm your tank inhabitants. A note on bulk Potassium Chloride, it may sometimes contain trace amounts of phosphate, so consider your sources.
I see a future where potassium dosing and testing is just regular as calcium and nitrate testing. People will talk about NPK ratios in reef forums. Start your future now. Explore potassium.