Potassium is a key element but more so for SPS corals. Low potassium can lead to color fading, lower growth and even STN. All salts contain potassium but usually at around 300ppm, natural sea water is around 380ppm so usually dosing is a good idea if you plan on keeping any SPS corals, otherwise regular water changes should be sufficient.
Also I dose many things without being able to test for it, these include amino acids, PotassiumIodide Fluoride, Pohl's B-Balance, and other trace element supplements. I believe these additives play a huge role in my coral health and vitality and I know many others who claim the same. Sometimes testing isn't needed, you can dose lower amounts and observe how the livestock responds and go from there. Besides I think it would be foolish to put too much trust in any test kit, even quality test kits will often give false readings from time to time.
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