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Potassium (K) comes from the word "potash", as potassium was first isolated from potash. Potassium is a soft silvery-white metallic alkali metal that occurs naturally bound to other elements in seawater and many minerals.
Potassium is an essential element for all living organisms and in the aquarium hobby this element is naturally supplied to aquatic animals by supplying good quality food.
In planted aquariums potassium is classed as a macro-nutrient for plants and this can be in short supply for some plants, so Potassium nitrate and Potassium sulphate is often added when practising 'PMDD' fertilisation.