Why water care matters
A cold plunge is a 50-gallon petri dish. The water is cold (which slows but doesn't stop microbial growth), dark (which algae loves), and constantly in contact with human skin (which sheds bacteria, oils, and dead cells). Without active water care, your plunge will go from crystal clear to slimy green in 10–14 days. With proper care, the same water can stay fresh for 3–6 months.
Water care isn't glamorous, but it's the difference between a plunge you use daily and one you avoid because it smells like a swamp. It's also a health issue — plunging in contaminated water can cause skin infections, ear infections, and in rare cases, serious illness from pathogens like Pseudomonas or Legionella.
The good news: water care is simple once you understand the three things you're managing. And it takes about 10 minutes per week.
The three things to manage
Cold plunge water chemistry boils down to three variables:
- Microbial load: Bacteria, algae, and biofilm. Managed by sanitation (ozone, chlorine, or both).
- pH balance: How acidic or basic the water is. Target 7.2–7.8. Outside this range, sanitizers work poorly and skin/eye irritation occurs.
- Particulate matter: Skin cells, hair, dust, debris. Managed by filtration.
Miss any of the three and you'll have water problems within a week. Manage all three and your water stays clean for months.
Ozone: the DIY standard
Ozone (O₃) is the preferred sanitation method for cold plunge. It's a gas you inject into the water, where it oxidizes organic contaminants (bacteria, viruses, biofilm) without leaving chemical residue. Ozone breaks down into plain oxygen within minutes — no smell, no skin irritation, no chlorine byproducts.
The standard setup is an inline ozone generator plumbed into your chiller's return line. The generator produces ozone gas, which is sucked into the water flow through a venturi injector. The ozonated water returns to your tub, sanitizing as it circulates.
The Coospider Ozone Generator is the most popular DIY choice — $45, produces 300 mg/h of ozone, enough for tubs up to 100 gallons. Run it 30 minutes, 3 times per week. That's it. For larger tubs (100+ gallons), step up to the Fuceter 1000 mg/h unit.
Coospider Ozone Generator
300 mg/h — perfect for 50-100 gallon tubs. Run 30 min × 3×/week.
Check Price →Fuceter 1000 mg/h
High-output for large tubs (100+ gal). Heavy-duty stainless body.
Check Price →Ozone gas is toxic to breathe at high concentrations. Your generator must inject ozone into the water, not into the air. Never operate an ozone generator in an unventilated indoor space, and never use a unit that vents ozone gas directly into the room. The units we recommend are all inline water-injection models.
Chlorine: the backup
Most serious DIYers run ozone as primary sanitation plus a small chlorine residual as backup. The chlorine handles anything the ozone misses (ozone has a short half-life in water — about 20 minutes — so it doesn't provide continuous protection).
For cold plunge, use 1–2 ppm of chlorine — much lower than a typical hot tub (3–5 ppm) because cold water slows microbial growth. Test weekly with 5-way test strips. If you smell chlorine on your skin after plunging, your level is too high.
If you prefer to avoid chlorine entirely, the alternative is PoolRX mineral algaecide — a copper/silver ion cartridge that drops into your filter pump. It's not as effective as chlorine for bacteria control, but combined with ozone it's sufficient for most residential cold plunge use.
Filtration
Your chiller probably has a built-in mesh filter — that catches large debris (leaves, hair) but won't catch fine particulates (skin cells, dust). For real filtration, you need a cartridge filter pump.
The Intex C1500 Cartridge Filter Pump is the standard DIY choice — $89, 1500 GPH flow rate, uses replaceable paper cartridges (replace every 2–4 weeks at $5 each). It's powerful enough for 50–100 gallon tubs and quiet enough for indoor use.
Run your filter pump 4–8 hours per day (most people run it on the same timer as their chiller). Clean the cartridge weekly by hosing it off; replace it monthly. A dirty filter restricts water flow, which reduces chiller efficiency and creates dead zones where bacteria thrive.
Testing your water
Test your water weekly with 5-way test strips. The five parameters:
| Parameter | Ideal Range | What to do if off |
|---|---|---|
| pH | 7.2–7.8 | Add pH Up (sodium carbonate) or pH Down (sodium bisulfate) |
| Total alkalinity | 80–120 ppm | Add baking soda to raise; sodium bisulfate to lower |
| Chlorine | 1–2 ppm | Add chlorine tablet or shock; dilute with fresh water if too high |
| Bromine | 2–4 ppm | Only if using bromine instead of chlorine |
| Hardness | 150–400 ppm | Add calcium hardness increaser; too hard = scale, too soft = corrosive |
Test strips cost about $14 for 150 strips — a 3-year supply. There's no excuse for not testing weekly.
When to drain & refill
Even with perfect water care, you'll eventually need to drain and refill. Signs it's time:
- Water won't clear even after filtration and shocking
- Total dissolved solids (TDS) above 1500 ppm (test with a TDS meter, $10 on Amazon)
- Smell that doesn't resolve after ozone treatment
- Cloudiness that doesn't resolve after 24 hours of filtration
- It's been 4–6 months since last drain (the practical maximum)
To drain: turn off chiller and pump, attach a garden hose to your bulkhead drain fitting, drain to a safe location (not storm drains — chlorinated water harms aquatic life), wipe down the interior with a 10% bleach solution, rinse, refill. Total time: 60–90 minutes. Mark your calendar for the next drain date.
Weekly maintenance checklist
Once your system is set up, ongoing maintenance takes about 10 minutes per week. Here's the checklist:
- Test water with 5-way strips (Monday)
- Adjust pH/chlorine as needed (Monday)
- Run ozone generator 30 min × 3 sessions (Mon/Wed/Fri)
- Inspect filter cartridge, hose off if dirty (Sunday)
- Skim surface debris with a fine net (as needed)
- Check chiller for unusual noise or vibration (Sunday)
- Verify water temp with independent thermometer (Sunday)
- Replace filter cartridge monthly
- Drain and refill every 4–6 months
For more detail on specific aspects of water care, see our ozone vs chlorine comparison and our water care FAQ. For step-by-step ozone installation, see our ozone generator installation guide.