Pool water turns green because of algae in the water. Algae can grow rapidly, especially when it’s warm like during the summer, which is why it can surprise you overnight. This is usually caused by a lack of circulation, high concentration of heat from sunshine and coupled along with chemical imbalance. Liquid chlorine only lasts for a couple of days by itself but by properly balancing the chemicals in your pool with stabilizers and calcium, running your pool a minimum 8 hours a day during sunshine, your pool will greatly improve, if not be perfect.