Abstract: A chemical composition for use in purifying water used in swimming pools having surfaces that are comprised of materials that do not react with or dissolve in water, comprising for 2.5 kg of the chemical composition, 1.25 kg of potassiumpersulphate, 300 g of sodium bisulfate, 250 g of ammonium chloride, 100 g of aluminum sulfate, 250 g of sodium bicarbonate, 250 g of calcium chloride and 100 g of ethylendiamine tetra acetic acid (abbreviated EDTA). The potassiumpersulfate and the ammonium chloride are effective in disinfecting water to be purified and in eradicating algae and bacteria therein. The sodium bisulfate, sodium bicarbonate and the calcium chloride are effective in balancing the pH of water to be purified to a pH value within an optimum range between 7.2 and 7.6. The aluminum sulfate serves as a flocculation aid and the copper sulfate is effective in the eradication of black algae. The EDTA is effective to complexing heavy metals in the water.