Cold plunge maintenance, without the guesswork

It depends entirely on your setup, which is why no honest answer is a single number. A tub with filtration and a sanitiser held at the right level, used by one person, can go weeks between changes; an unfiltered tub with no treatment, used by several sweaty people, might need refreshing weekly or sooner. The variables are how many people use it, how sweaty they are going in, whether you shower first, whether you run a filter, and whether you treat the water at all. Rather than invent an interval, watch the water: clarity, smell and the feel of the surfaces tell you more than a calendar does. Test strips turn that from a guess into a reading.

Cloudy usually means particulate and body oils that filtration has not caught - sweat, skin, dust - and it is the first sign your filter or your water changes are falling behind. Green means algae, which needs light and warmth and time; it is more common in outdoor tubs, warmer water and tubs left uncovered. Cold water slows biological growth but does not stop it, and the moment a tub warms up between sessions the odds rise. The fixes are the same routine in different proportions: filter more, treat consistently, keep it covered and out of the light, and change the water when the routine can no longer keep up.

Cold slows growth; it does not sterilise. Bacteria and algae grow more slowly near freezing but they still grow, especially as the water warms between sessions and as sweat and skin feed them. Whether you need a treatment depends on how you use the tub: shower-first, single-user, filtered, covered setups can sometimes get by on filtration and frequent changes, while shared or unfiltered tubs benefit from a sanitiser. What matters is that a treatment only works if you can dose it and hold it, which means knowing what is in the bottle - and, as our water-care page found, most cold-plunge-branded bottles will not tell you.

We deliberately do not publish a target number, and here is why. Safe recreational-water targets for pH and sanitiser residual live in public-health and state regulatory documents, and when we tried to verify a specific figure against that primary text, the sources were not retrievable. Rather than repeat a number we cannot stand behind, we state the principle - a residual is a measured amount of a known sanitiser held at a controlled pH - and point you to your own health authority and to your chiller or treatment maker's instructions. Anyone quoting you an exact ppm for a cold plunge should be able to show you where it comes from.

No - they do different jobs, and you often want both. A filter removes what you can see and feel: hair, skin, grit, the particulate that makes water cloudy. A sanitiser handles what you cannot see: bacteria and the beginnings of algae. Filtration is not sanitation and sanitation is not filtration. A submersible filter with a UV lamp, like the one we rank first on the water-care page, blends the two, but even that is clarity plus UV rather than a held chemical residual. Match the tools to how dirty your water actually gets.

Keep the chiller's own filter clean - a clogged filter makes the compressor work harder and cool worse - and follow its manual for descaling and winter care, because the manufacturer's instructions outrank any general advice, including ours. Keep the tub covered when idle to keep out light, debris and warmth. Rinse anything that touches treated water, including neoprene socks and covers, in fresh water. And drain fully before a hard freeze if the tub or lines are not rated to stay wet through it. None of this is exotic; it is the difference between gear that lasts years and gear that does not.