Once any hot tub has been filled, an imaginary clock begins ticking, counting down to the day when the water will have to be replaced.
It's a costly but necessary process — costly due to the expense of sewering the old water and buying new, heating it to a user-friendly temperature and filling it with sanitizer and (in some cases) appropriate supplements. And then there is the cost of labor.
It's necessary because spa water contains more than just water. It's a high-performance solvent for a diverse set of compounds — some of these enter the spa in the sourcewater, others are added intentionally in water care products and still others unintentionally when the human body slides below the surface and begins to release dead skin, sweat and other greasy substances into the surrounding medium.
Over time the soup becomes imperceptibly thicker. This occurs partly due to evaporation as individual H2O molecules become airborne, leaving impurities behind in an ever more concentrated solution. But the primary cause is that over time, more and more bathers leave behind their wastes, and more chemicals are added to maintain pH, ORP, etc., and these simply build up.
At some point, water quality becomes difficult to manage. Its chemistry becomes unresponsive — sort of like driving a vegetable truck. Specific problems crop up, letting you know it's time to drain.