Cold plunge water care: filters, ozone, and how often to change the water
The thing nobody mentions before you buy a cold plunge is that you're buying a small, cold swimming pool. Here's how to keep it clean without turning it into a chemistry project.
Why plunge water gets dirty
Every dip adds skin cells, sweat, body oils, sunscreen and whatever was on your feet. Cold slows bacterial growth but doesn't stop it, and a covered tub in a warm garage can turn cloudy in a week without treatment. Clear water is partly about health and mostly about not wanting to climb into a soup.
The three layers of water care
Keep stuff out
Rinse off before you get in; a 30-second shower removes most of what would end up in the tub. Keep it covered when not in use. Wear clean swimwear or none. Don't plunge right after applying lotion or sunscreen.
Filter continuously
Most chiller units include a pump and a cartridge filter that cycles the water constantly. Rinse the cartridge weekly (hose it off) and replace it every 1-3 months, or sooner if flow drops. A 20-micron filter catches debris and skin; some owners add a fine pre-filter sock on the intake.
Sanitize
Filtration removes particles; sanitizing kills bacteria. Options, roughly in order of how most owners run them:
- Ozone (built into Plunge, Morozko and many others): injected automatically, no taste, no residue. Effective but short-lived, so it's usually paired with a light dose of something else.
- UV (Sun Home and others): water passes a UV lamp that kills microbes. Same "no chemicals" appeal; bulbs last about a year.
- Hydrogen peroxide: a cup or so of 3% peroxide a few times a week, or a non-chlorine shock weekly, keeps water fresh without a chlorine smell. The most common DIY approach.
- Chlorine or bromine: hot-tub tablets or liquid chlorine in small doses work and are cheap, but many people dislike the smell on skin. If you use them, test strips keep you in range (1-3 ppm free chlorine).
How often to change the water
| Setup | Change water |
|---|---|
| Tub with chiller, filter, and ozone or UV, one user, rinses first | Every 4-8 weeks |
| Same, two or more users | Every 3-4 weeks |
| Filter but no sanitizer | Every 1-2 weeks |
| Stock tank or bathtub, ice only, no filter | Every 1-3 uses |
| Outdoor, uncovered, or heavy pollen season | Shorten any of the above |
Change it sooner if it's cloudy, smells, feels slimy, or foams when the pump runs. Those are the signs the biofilm has won.
Cleaning the tub itself
When you drain it, wipe the walls with a diluted white vinegar solution or a mild non-abrasive cleaner, rinse, and dry. Run a tub-cleaner or a strong peroxide solution through the plumbing every few months to clear biofilm from the lines (the same products hot-tub owners use for "pipe flush" work). Refill through a hose filter if your tap water is hard; it keeps scale off the chiller.
Winter and summer
- Summer: the chiller works hardest and water warms fast when uncovered; keep the lid on and expect higher electricity use. Algae needs light, so a cover is your best defense.
- Winter outdoors: most chillers keep water from freezing by circulating, but check the manual's minimum ambient temperature. Insulate exposed hoses. If you're away, drain it.
A simple weekly routine
Rinse the filter, dose peroxide or check the ozone/UV indicator, skim anything floating, wipe the waterline, check the temperature. Five minutes. Monthly: inspect the cartridge, clean the cover. Every 4-8 weeks: drain, wipe, refill. Do that and the water stays clear enough to see the drain.
Shopping for a tub? Filtration and sanitation are two of the things we weigh in the best cold plunge tubs.