Green water in a swimming pool or hot tub is caused by green algae, which develops due to poor water circulation, lack of proper cleaning, poor sanitation practices or low pH levels. Such conditions can cause green algae blooms to take over a pool within days or even hours.
Although it can spread quickly, green algae can also be eliminated quite easily, unlike the other common forms of pool algae. The best option for getting rid of green algae in a pool is to shock the water with a large dose of chlorine followed by an appropriate amount of algaecide.
The amount of chlorine needed depends on the extent of the algae bloom, which can be judged by the color of the water. If the water is light green, it should only require a double shock, whereas dark green water usually requires a triple shock, and black-green water requires at least a quadruple shock.
The chlorine and algaecide should kill the algae and turn the pool a cloudy blue or gray color, as this is the color algae turns when it dies. Once the pool has turned this cloudy color, the filtration system should be run constantly until the water returns to its normal, clear state.