You don’t even have to change the water in an outdoor pool because it is completely renewed every 1.5 years just through rainwater. For example, a 4x8 metre basin holds 37,500 litres of water. With the average annual precipitation in Germany, 25,000 litres of rainwater are fed into the basin every year. In 1.5 years, the tank contents are completely renewed once. In the case of an indoor swimming pool, this is naturally somewhat different because freshwater must be supplied continuously just to compensate for evaporation. The water should be completely changed every three to five years because the chloride content in the water also increases over time.