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Quick answer
A filter catches hair, skin flakes and grit. It doesn't kill anything. For a DIY tub, the working setup is a cartridge filter pump that moves the tub's full volume at least once an hour (San Francisco's rule for business plunges), plus a sanitizer that stays in the water. Ozone and UV-C kill microbes only in the water passing through them and leave nothing behind, so they support chlorine, bromine or peroxide rather than replace them, unless your tub's maker specifies its own system. Swap cartridges on schedule: every 2 weeks for Intex pool cartridges, monthly for Icetubs and Kohler.
Four jobs, five tools.
Most confusion about plunge water comes from expecting one device to do every job. Here's what each piece actually does.
What each piece of cold plunge water equipment does
| Tool | What it does | What it doesn't do | Upkeep |
|---|---|---|---|
| Cartridge filter | Traps particles down to its rating; Icetubs and Finnmark both use 5-micron cartridges | Kill germs or leave anything in the water | Rinse and replace on schedule (table below) |
| Filter pump | Pushes water through the filter, and through a chiller, UV or ozone if you have them | Clean anything by itself; it also adds heat | GFCI outlet, unplug before service, clean intake |
| Ozone generator | Oxidizes organics and kills microbes in the water passing the injector | Stay in the water; EPA says ozone decomposes rapidly | Ventilation, ozone-rated parts |
| UV-C clarifier | Kills microbes in water flowing past the lamp | Treat anything that stays in the tub | Clean the quartz sleeve, replace the lamp |
| Chemical sanitizer | Keeps a residual working in all the water, all the time | Remove particles or body oil | Test twice a week |
Cartridge filters: size, micron and swap schedule.
A micron is a thousandth of a millimeter. A lower rating catches finer particles and loads up sooner. Icetubs specifies a 5-micron polypropylene cartridge; Finnmark's SoulCold runs a 5-inch, 5-micron pleated main filter plus a separate carbon filter and a hairnet filter. Intex pool pumps take their own Type A or Type H cartridges.
Cartridge check and replacement schedules from each maker
| Filter | Size or rating | Check or rinse | Replace |
|---|---|---|---|
| Icetubs IceBarrel and IceBath | 5 micron, 5.9 in diameter x 4.2 in high | Inspect weekly for flow and clarity | Monthly for private use, weekly for light professional use |
| Finnmark SoulCold (and the FDC-1A chiller) | 5-inch, 5-micron pleated main filter, plus carbon and hairnet filters | Every 10 uses, including the hairnet filter | The warranty section says the paper core filter should be replaced every 30 days |
| Kohler x Remedy Place Ice Bath | Kohler replacement filter (size not listed) | As needed | Every month or as needed; drain the bath completely first |
| Intex Type A (C530 pump) | 8 in high x 4.25 in wide | Hose it off, per Intex | Every two weeks |
Three habits from those manuals apply to any tub. Keep a spare on the shelf, because Icetubs says the engine must never run without one and doing so voids its warranty. Rinse with a hose unless your maker says otherwise. And replace on the calendar, not just on looks: a rinsed cartridge isn't a new one, and a clogged one cuts flow through everything downstream. If your SoulCold water turns tea-colored, that's tannin from the cedar backrests; Finnmark says the carbon filter helps absorb it and that it fades over several water changes.
Filter pumps for DIY tubs: match GPH to gallons.
San Francisco's health department says cold plunge water at businesses must fully cycle through filtration and disinfection at least once per hour. That's a sensible floor at home, too: the pump's real flow in gallons per hour should be at least your tub's volume in gallons. Two turnovers an hour is comfortable.
Minutes for one full turnover at two common filter-pump flow rates
| Tub size | Flow for 1 turnover an hour | At 300 GPH | At 475 GPH (Intex C530 system rating) |
|---|---|---|---|
| 80 gallons | 80 GPH | 16 minutes | 10 minutes |
| 100 gallons | 100 GPH | 20 minutes | 13 minutes |
| 125 gallons | 125 GPH | 25 minutes | 16 minutes |
| 150 gallons | 150 GPH | 30 minutes | 19 minutes |
| 200 gallons | 200 GPH | 40 minutes | 25 minutes |
Box numbers are best-case. Intex rates the C530 at 530 GPH at the pump and 475 GPH installed on its own pool; every extra fitting, foot of hose, UV chamber and loaded cartridge takes more off. Even so, almost any filter pump clears the once-an-hour floor on a plunge-size tub. The more common mistake is oversizing: more heat in the water, and more flow than a UV unit is rated for.
Pumps add heat. The electricity a pump draws ends up as heat, most of it in the water with a submersible pump and part of it with an external one. Using the same physics as our ice calculator: 100 watts running around the clock is about 8,200 BTU a day, enough to melt roughly 52 pounds of ice in a tub held near 45°F. In an ice-only tub, put the pump on a timer for a few hours a day, enough for several full turnovers, rather than running it nonstop, and check the watt rating on your pump's label.
For pump types, priming and submersible versus inline, see the pump guide.
Safety: Plug any pump, UV unit or ozonator into a GFCI-protected outlet, keep connections dry and off the ground, and unplug a DIY rig before you get in. Intex's manual says to always unplug its filter pump before removing, cleaning or servicing it, and describes the pump as for use with storable pools only, so plumbing one into a stock tank is your own engineering.
Ozone: strong, brief, and best with the lid open.
An ozone generator turns oxygen into ozone and injects it into the water stream. EPA calls ozone more effective than chlorine at destroying viruses and bacteria. The catch is in the same fact sheet: ozone has limited solubility, decomposes rapidly, and decomposes faster in water than in air. It works on the water passing the injector and is gone soon after, so nothing protects the tub between cycles. San Francisco tells businesses that plunges using UV or ozone should supplement with chlorine or bromine.
Cold slows it too. EPA's tables give ozone a contact time about four times longer at 41°F than at 77°F, the same ratio as chlorine.
- Built-in ozone is common. The Icetubs engine has a built-in ozonator rated at 200 to 300 mg per hour that runs about half the time the pump does, and Icetubs says to keep the engine running at least 6 hours a day. Finnmark's SoulCold and FDC-1A chillers list an ozone generator too.
- Open the lid before you get in. Ozone that doesn't dissolve bubbles out at the surface. EPA calls ozone gas extremely irritating and possibly toxic, and its indoor-air page lists chest pain, coughing and throat irritation from breathing it; OSHA's workplace limit is 0.10 ppm averaged over 8 hours. A closed cover holds that gas exactly where your face goes, so let the tub air out for a few minutes first, especially indoors.
- Parts have to tolerate it. EPA describes ozone as very reactive and corrosive, requiring materials such as stainless steel. A tub with ozone built in has had that worked out. A DIY add-on needs tubing and seals the ozonator's maker approves.
- Pair it carefully. Leisure Time says its Spa 56 chlorine works alongside ozone systems, and Icetubs pairs its ozone with AquaFinesse. Hydrogen peroxide is the one to handle with care: peroxide speeds ozone's breakdown into short-lived hydroxyl radicals (EPA's disinfectant guidance calls the pairing "peroxone"), so the two use each other up. Some chiller makers do run modest peroxide levels alongside their ozone; the peroxide guide lists their numbers. Use your chiller maker's figure, not a generic chart.
We don't list a DIY ozonator here. Spa ozonators are built to plumb into circulation through an injector, and bubbling ozone into the tub through an air stone puts the off-gas right at your face.
UV-C: kills what flows past, nothing else.
A UV-C clarifier is a lamp inside a sealed chamber in the plumbing. Water that flows past the lamp gets a dose; water that stays in the tub doesn't. Three things decide whether it earns its place:
- Flow rating. Each unit has a maximum flow. Slower flow means more exposure per pass. Tetra rates its 5-watt UVC-5 at 330 GPH; the Intex C530's 475 GPH system rating is over that, so on a C530 the water gets less time under the lamp than Tetra designed for.
- Clear water. Put the UV unit after the filter so the lamp sees filtered water, and clean the quartz sleeve around the lamp. Kohler, which builds UV into its ice bath, says to clean the UV bulb every 6 months.
- Lamp life. Tetra's UVC-5 lamp is rated for up to 11 months of continuous operation, so budget a replacement each year.
Even with built-in UV, Kohler's manual still lists 2 to 4 ppm free chlorine for its ice bath. That tells you how the maker of a UV-equipped plunge thinks about it: support, not a stand-alone sanitizer.
FDA warns that direct exposure of skin and eyes to some UV-C lamps can cause painful eye injury and burn-like skin reactions, and says never to look at a UV-C lamp, even briefly. Inline units keep the lamp in a housing. Submersible UV lights sold for ponds put the lamp in the water you sit in; run one only when nobody's in the tub, or skip it.
How filters, ozone and UV pair with a sanitizer.
Sanitizer pairings by setup
| Setup | Pair with | Don't pair with |
|---|---|---|
| Icetubs (built-in ozone, 5-micron filter) | AquaFinesse, dosed weekly per the manual | Chlorine, bromine, acids or salts, per Icetubs |
| Kohler x Remedy Place (built-in UV and filter) | Free chlorine at 2 to 4 ppm, or a non-chlorine sanitizer used per its maker's instructions | Anything outside Kohler's manual |
| Chiller with ozone (Finnmark SoulCold, FDC-1A) | The sanitizer Finnmark approves for your unit, in writing | Hydrogen peroxide, unless Finnmark approves it |
| DIY tub with a filter pump and UV-C | Spa chlorine at 2 to 4 ppm, bromine at 2 to 4 ppm, or peroxide started at 50 ppm (see the peroxide guide) | Any second sanitizer system |
| DIY tub with no filter | A sanitizer residual and a fresh fill every 3 to 7 days | Waiting for the water to look bad |
Doses and targets live on the chlorine guide and the peroxide guide; the routine and drain schedule live on the water care guide.
Brand-name plunge, brand-name filter.
Model-specific tubs need model-specific cartridges, and Icetubs and Finnmark both tie warranty terms to genuine parts. Sauna Marketplace carries the exact cartridges for three systems:
- Icetubs 5 Micron Water Filters at Sauna Marketplace: six genuine cartridges for compatible IceBarrel and IceBath systems.
- Finnmark 5 Inch Pleated Filters at Sauna Marketplace: the pleated main filter for the SoulCold and for the FDC-1A chiller supplied with SaunaLife S1 packages.
- Finnmark SoulCold Carbon Filters at Sauna Marketplace: the separate carbon stage in the SoulCold.
Match the cartridge and housing on your unit before ordering. The SaunaLife S2's AP-1 chiller is a different configuration and needs its own compatibility check.
For DIY tubs: pump, cartridges and UV.
Best for filtering a DIY tub
INTEX C530 Krystal Clear Cartridge Filter Pump
- Flow: 530 GPH at the pump, 475 GPH system rating
- Cartridge: Intex Type A (29000E)
- Hoses: 1.25-inch (32 mm) fittings
- Power: 110 to 120V with GFCI, per Intex
- Warranty: 2 years
The cartridge sits in a housing outside the tub, so changing it doesn't mean reaching into cold water, and the pump's GFCI is built in. At its system rating it turns a 100-gallon tub over about every 13 minutes, far past the once-an-hour floor, so a few hours of running a day is plenty. Type A cartridges are sold almost everywhere pool supplies are.
Skip it if: you don't want to cut 1.25-inch fittings through the wall of your tub (Intex designs it for its own storable pools), or you plan to add a UV unit rated under 475 GPH. Amazon also lists a newer 26603EP version with a built-in shutoff timer.
Best for C530 owners
Intex Type A Easy Set Pool Filter Cartridge, 12 Pack
- Size: 8 inches high x 4.25 inches wide
- Count: 12 cartridges
- Part numbers: 29000E / 59900E
- Swap: as often as every two weeks, per Intex
Twelve genuine cartridges last about 24 weeks at Intex's two-week schedule. Hosing one off between swaps keeps flow up; the swap itself goes on the calendar, not on looks.
Skip it if: your pump takes Type H or another cartridge, or you own an Icetubs, Finnmark or Kohler tub, which use their own filters.
Best for small, slow UV loops
Tetra Pond GreenFree UV Clarifier UVC-5
- Lamp: 5 watts, rated up to 11 months of continuous operation
- Maximum flow: 330 GPH
- Connections: 3/4, 1 and 1-1/4 inch barbed
- Install: out of the water only; UL listed
- Warranty: 3 years, lamp excluded
An inline unit keeps the UV-C lamp sealed in a housing with a stainless reflective insert, outside the water you sit in. The 1-1/4 inch barb matches Intex-size hose. On a 100-gallon tub at 300 GPH, the equivalent of the whole tub passes the lamp three times an hour.
Skip it if: your pump pushes more than 330 GPH through it, as the Intex C530 does at its rating; Tetra's 9-watt UVC-9 is rated to 900 GPH but showed out of stock on Amazon at publication. And skip it if you were hoping to drop chemicals: UV leaves no residual.
FAQ
What's the best filter pump for a cold plunge?
For a DIY tub, a cartridge filter pump whose real flow in gallons per hour is at least the tub's volume in gallons. The Intex C530 clears that easily for plunge-size tubs and has a built-in GFCI; brand-name plunges should use their own filter system.
Do I need a filter for my ice bath?
No, but without one, nothing removes what you leave behind, so you drain far more often: every 3 to 7 days is our rule of thumb. The water care guide covers how to keep ice bath water clean without a filter.
How often should I change a cold plunge filter?
Follow the maker: every two weeks for Intex pool cartridges, monthly for Icetubs (weekly in light professional use) and Kohler, and Finnmark says the paper core filter every 30 days, with checks every 10 uses.
What micron filter should a cold plunge use?
Icetubs and Finnmark both specify 5-micron cartridges. Finer catches more and loads up sooner, so pair a fine cartridge with a pre-filter or more frequent rinsing.
Is ozone safe in a cold plunge?
Dissolved ozone breaks down fast, but the gas is a lung irritant, and it collects under a closed cover. Open the lid and let the tub air out before you get in, and use only parts rated for ozone.
Can a UV sanitizer replace chemicals?
Not by itself. UV-C treats only the water flowing past the lamp and leaves no residual, which is why San Francisco tells business plunges with UV or ozone to add chlorine or bromine, and Kohler still lists a chlorine range for its UV-equipped ice bath.
Can I run a water chiller and a filter together?
Yes, and most packaged plunge chillers already include a filter housing; put the filter ahead of the chiller so debris doesn't reach it. Our chiller guide covers what each one includes.