Potassium ( K) is a macro nutrtion for aquarium plants and even important as e.g. Iron. Especially in areas with soft water, there is often a potassium deficiency in the aquarium. Potassium deficiency is often confused with iron deficiency, but in case of lack of potassium mostly the whole leaf is very pale - almost white. In case of Iron deficency there are stiil green nerves visible. Use potassium with Iron. Best is to use potassium together with iron, to exclude defieciencys and to be sure both elements are present.