Why might a swimming pool be drained completely?

Draining a swimming pool completely can be necessary to address severe algae problems or to perform extensive repairs. When there is a significant algae infestation, simply treating the water with chemicals may not suffice, as the algae can cling to surfaces and persist despite water treatments. A complete drain allows for thorough cleaning and scrubbing of the pool's walls and floor, ensuring that all algae is removed effectively. Additionally, draining the pool may be required for repairs that cannot be conducted with water in the pool, such as fixing structural issues or replacing tiles.

While maintaining water quality during the off-season and potentially increasing the pool's lifespan are valid considerations, they do not typically necessitate complete draining. Changing water temperature is also not a reason to drain the pool entirely, as temperature adjustments can be made with water still in the pool through heating mechanisms. Therefore, addressing severe algae issues or performing repairs is the most compelling reason for complete drainage.