Potassium occurs at a concentration of about 400 mg/L in full-strength natural seawater. It is an essential element for metabolic function of all living cells. Sometimes artificial seawater mixes are deficient in potassium, and some reef aquariums lose potassium over time, due to filtration methods and extraction of potassium from the water by marinelife. For corals, potassium deficiency is characterized by loss of color, slow growth, or loss of tissue, especially in small polyped stony corals. Use Two Little Fishies Potassium Concentrate to raise the potassium concentration in the aquarium when it falls below 400 mg/L. Potassium supplementation is especially beneficial in low-nutrient reef aquariums, and is known to promote the development of colorful pigments in Montipora, Acropora, Stylophora, Pocillopora, & Seriatopora spp. 500 ml