Use circulation plus filtration, keep the tub covered, control sanitizer according to the equipment maker, and replace water when chemistry or clarity can no longer be maintained.
Filtration removes particles, not all microbes
A cartridge or other mechanical filter captures debris and helps keep water clear. It should not be treated as complete disinfection by itself.
Clean or replace the filter on a schedule appropriate to use and loading.
Sanitation still matters
If the plunge is used repeatedly without a full drain after every session, a compatible sanitizer or validated sanitation system is generally needed.
Follow the tub/chiller manufacturer because materials and seals may not tolerate every chemical at every concentration.
Cover and shower habits reduce load
A fitted cover reduces dust, leaves and sunlight exposure. Showering before entering removes a large amount of sweat, cosmetics and organic material that would otherwise consume sanitizer and foul filters.
Water-change interval is not one number
The right interval depends on bather load, filtration, sanitation, temperature and whether chemistry remains controllable.
Replace the water sooner if odor, cloudiness, biofilm or persistent chemistry problems develop.