Hot Tub Treatment

Water chemistry, sanitizer strategy, biofilm remediation, and Pseudomonas folliculitis response. Practical numbers, not fluff.
Updated 2026-05-20 · Assumes residential spa, 300-500 gal

Target Chemistry Ranges

ParameterLowIdealHighNotes
pH< 7.27.4 - 7.6> 7.8Drifts up over time from CO₂ off-gassing
Free Chlorine (FC)< 1 ppm3 - 5 ppm> 10 ppmLower if ozone working; never bathe with 0
Bromine (alt to Cl)< 2 ppm3 - 5 ppm> 8 ppmBetter choice without ozone
Total Alkalinity (TA)< 60 ppm80 - 120 ppm> 180 ppmLow TA = pH bouncing; high = scale risk
Calcium Hardness< 100 ppm150 - 250 ppm> 400 ppmBelow 100 = foaming, equipment etching
Cyanuric Acid (CYA)0 ppm0 - 20 ppm> 30 ppmKeep LOW in spas; binds Cl in hot water
Temperature< 95°F97 - 104°F> 104°F104°F max for adults; 97-100°F for kids
Why CYA stays low in a spa (vs. high in a pool) In 100°F+ water, CYA-bound chlorine is dramatically less effective as a sanitizer. The "stabilizer" you want in an outdoor pool is a liability in a hot tub. Use unstabilized chlorine (bleach or cal-hypo) rather than dichlor or trichlor.

Test Strip Interpretation

Reading a standard 4-way AquaChek-style strip (FC + pH + TA + CYA):

  1. Dip the strip in still water for 1 second, remove, hold horizontal (don't shake).
  2. Wait 15 seconds exactly. Reading too early or late gives bad results.
  3. Compare to bottle chart in natural light, not under yellow indoor light.
  4. End pad = pH. Pad nearest handle = CYA. (Standard layout.)
PadColor shiftWhat it tells you
pHYellow → orange → redYellow = acidic, red = alkaline
FCWhite → light purple → dark purpleWhite = no sanitizer (do not soak)
TAYellow → green → tealYellow = no buffering capacity
CYAYellow → orange → magenta → purpleYellow ≈ 0 ppm (correct for spa)
Strip limitations Strips are accurate to within roughly ±0.5 pH, ±20 ppm TA, ±1 ppm FC. For precise diagnosis (e.g., diagnosing chlorine lock), use a Taylor K-2006 drop test kit instead.

Sanitizer Strategy Decision

Q1: Does your spa have a working corona-discharge ozonator?
Yes →
Run chlorine (bleach). FC residual 1-2 ppm is enough; ozone handles primary oxidation.
No (or unknown) → Q2
Q2: Do you run the tub at 102°F+?
Yes → Switch to bromine. Better stability in hot water, less off-gassing, less daily fiddling.
No (running 97-100°F) →
Either works. Bleach is cheaper and simpler if you'll dose daily.
Ozonator lifespan check

Corona-discharge ozone cells typically last 2-5 years. Test:

  1. Run circulation pump only (jets off), cover on, 30+ minutes.
  2. Lift cover and sniff near lowest jet. Sharp chlorine-like bite = working.
  3. No smell = dead. Replacement modules run $150-300 and are usually plug-and-play.
Chlorine (bleach)BromineDichlor
Cost/month$5-10$10-15$10-15
Adds CYA?No goodNoYes bad in spa
Hot water stabilityModerateHigh bestModerate
Daily attentionYes (2-3 oz daily)Minimal (floater)Yes
Best forCooler spa, ozone-equippedHot spa, no ozoneAvoid in spa

Interactive Dosing Calculator

gallons
ppm    ppm
ppm    ppm
Baking Soda (TA up)
—
Costco sodium bicarbonate = same as "Alkalinity Up"
Bleach 8.25% (FC up)
—
Plain unscented Clorox. NO "splash-less" or scented.
Dichlor alternative (FC up)
—
Avoid in spas; builds CYA
Shock dose (10 ppm FC)
—
Use for weekly oxidation or post-heavy-use
Hyperchlorinate (30 ppm FC)
—
For biofilm remediation only. Do not bathe.
Ahh-some purge dose
—
16 oz liquid concentrate, 2 oz per 100 gal
Math basis Bicarb: 1.4 oz raises TA ~10 ppm per 1000 gal. Bleach 8.25%: 1 oz raises FC ~6 ppm per 100 gal. Dichlor 56%: 0.16 oz raises FC ~10 ppm per 100 gal (also adds ~9 ppm CYA).

Startup Protocol (New Fill / After Drain)

Critical: do not bathe within 24 hours of fresh fill A tub that sat empty harbors Pseudomonas biofilm in plumbing. First-fill water has zero sanitizer history. Wait until FC has been ≥3 ppm for 24 hours minimum.
Order matters

Adjust TA before pH. Bicarb raises both, so chasing pH first wastes chemicals. Same with shock — never add sanitizer when pH is below 7.0 (chlorine off-gasses as Cl₂) or above 8.0 (mostly inactive OCl⁻ form).

Biofilm Purge Protocol

When to purge: every water change (3-4 months), after any contamination event (folliculitis outbreak, cloudy water that won't clear), after extended downtime (winter storage), or when buying a used tub.

Why a regular shock isn't enough

Hot tub plumbing develops biofilm — a polysaccharide matrix that bacteria secrete to shield themselves from sanitizer. Chlorine kills planktonic (free-floating) bacteria in seconds but can't penetrate the biofilm matrix. A surfactant-based purge product is required to physically disrupt the matrix so chlorine can reach what's inside.

Product comparison
ProductFormEffectivenessCost/purge
Ahh-some (Unique Solutions / King Tech)Gel or 16 oz liquidBest$3-5
Oh YukLiquidGood (2-cycle)$4-6
Natural Chemistry Pipe CleanseLiquidModerate$5-8
TSP + bleach (DIY)Powder + liquidAcceptable$3-5
Full purge sequence (Ahh-some 16 oz liquid)
1Remove filter cartridge. Set aside in trash; do not reinstall.
2Add 2 oz Ahh-some liquid per 100 gal (8 oz for 400 gal). Pour into water near a jet inlet.
3Add 20 oz bleach 8.25% (per 400 gal) simultaneously to hyperchlorinate to ~30 ppm during the purge.
4Run all jets + air on high, 60 minutes. Expect heavy foam and brown/gray gunk surfacing — that's biofilm release.
5Drain completely via the spa's drain plug. Run pump dry briefly to clear lines.
6Wipe shell with 1:10 bleach solution. Pay attention to waterline, jet faces, filter well.
7Refill, install new filter, restart chemistry per Startup Protocol.
Heavy contamination = double purge For known pathogen events (folliculitis outbreak, after long winter storage), run two consecutive Ahh-some cycles separated by a drain/refill. Each cycle releases progressively less material — that's the visual confirmation it worked.

Pseudomonas Folliculitis Response

Pseudomonas aeruginosa folliculitis ("hot tub rash") presents 1-4 days after exposure as red, itchy bumps concentrated in swimsuit-covered areas. Self-limiting in 7-14 days but actively treatable to shorten duration and prevent worsening.

First-line interventions (do all)
InterventionMechanismProtocol
Chlorhexidine 4% (Hibiclens) Hospital-grade antiseptic, kills Pseudomonas on skin and superficial follicles Lather on affected areas in shower, 1-2 min contact, rinse. Once daily, 3-5 days. Avoid eyes, ears, mucous membranes.
Acetic acid compresses (1:1 white vinegar + water) Pseudomonas cannot survive at low pH — same principle as swimmer's ear treatment Soak washcloth, apply 10-15 min to affected areas, 2-3× daily until rash clears.
Wash all exposed fabric (hot + bleach) Prevents re-inoculation; Pseudomonas survives on damp fabric for days Swimsuits, towels, bath mats, robes. Hot wash, full detergent, 1/2 cup bleach if fabric allows. Sun-dry if possible.
Keep affected skin dry & uncovered Occlusion + moisture + warmth = the conditions that caused this Loose cotton, no tight elastic over rash, change if sweaty.
Do NOT do
  • No OTC topical antibiotics (Neosporin, bacitracin, triple-antibiotic). They don't cover Pseudomonas and may select for resistance.
  • No popping, picking, or scratching — introduces secondary Staph and drives bacteria deeper.
  • No shaving, waxing, depilatories on affected areas until fully resolved.
  • No pools, hot tubs, lake/river swimming, shared baths until rash is gone.
  • No sharing towels between affected individuals.
Escalate to physician immediately if: Fever > 100.4°F · expanding redness around any lesion (draw a Sharpie ring around the worst spot, check at 12 hr) · individual bump becoming firm/nodular/larger than ~5 mm · child or adult acting genuinely unwell (lethargy, deep pain rather than itch) · symptoms still worsening after 4-5 days of treatment. Oral ciprofloxacin or other anti-pseudomonal antibiotics may be needed.
Prevention going forward
  1. Shower with soap immediately after every soak — don't sit in a contaminated swimsuit.
  2. Maintain FC ≥ 3 ppm always; never bathe if test shows 0 ppm.
  3. Replace filter every 12 months or after any contamination event.
  4. Full Ahh-some purge every 3-4 months minimum.
  5. Drain and refill every 3 months (calculation: refill when TDS reaches roughly 1500 ppm above source water, or quarterly, whichever first).

Temperature Strategy

ScenarioSetpointChemistry Implications
Adult relaxation102 - 104°FHighest sanitizer demand, fastest pH drift up
Mixed family (kids)97 - 100°F~25% lower chlorine demand than 104°F; bromine advantage shrinks
Therapy / soak time only100 - 102°FStandard setpoint, predictable chemistry
Storage between uses (1+ weeks)Off or floor setpoint~3-4× longer chlorine residual; minimal heating cost
Decontamination wait periodOff / lowest possibleSlows bacterial growth, preserves chlorine
Arrhenius math Reaction rates roughly double per 10°C (18°F). Dropping from 104°F to 97°F cuts most chemistry reactions (including bacterial growth AND chlorine decomposition) by ~25-30%. Net: cooler water means longer-lasting sanitizer AND slower pathogen multiplication.
97-104°F is the Pseudomonas sweet spot Lower temperatures don't make a hot tub bacteria-free — they just slow growth. Sanitizer levels still need to be maintained at every setpoint above ~80°F. Running cooler is a chemistry win, not a sanitizer-skip license.

Troubleshooting Common Issues

SymptomLikely CauseFix
Cloudy water High bather load, dead chlorine, biofilm release, or high TA/calcium Test all parameters; shock to 10 ppm; clean filter; if persistent → purge plumbing
Foaming Body oils, soap residue, low calcium hardness, or surfactant accumulation Run purge product; raise calcium to 150+ ppm; long-term: rinse off before soaking
Strong chlorine smell Chloramines (combined chlorine), not free chlorine. Counter-intuitive: indicates UNDER-chlorination Shock to 10× combined chlorine reading; "breakpoint chlorination"
Rotten egg / musty smell Anaerobic bacteria in plumbing (biofilm) Full purge + drain + refill. Don't try to chlorinate through it.
Skin irritation, rash 1-4 days after use Pseudomonas folliculitis (see dedicated section) Hibiclens protocol + remediate tub before reuse
Itching during/right after soak High chloramines, low pH, or chemical sensitivity Verify pH 7.4-7.6, FC 3-5 (not higher), shock if combined Cl present
Green or pink-tinted water Green = algae (rare in covered spa); pink = Methylobacterium biofilm Shock + scrub + purge cycle
Chlorine won't hold (drops to 0 fast) Chlorine lock from CYA, ammonia from bather waste, or biofilm consuming sanitizer Test CYA; if >30 partial drain; if biofilm suspected → purge
pH keeps drifting up Normal CO₂ off-gassing. High aeration (waterfall, air jets) accelerates it Lower TA to 80 ppm; pH will be more stable. Reduce aeration when not in use.
Scale on heater / waterline High calcium + high pH + high temp = CaCO₃ precipitation Lower pH to 7.2-7.4, drain partial if Ca >400 ppm, descale with citric acid

Chemical Quick Reference

Brand-nameReal chemicalCheap sourcePer-lb cost
"Alkalinity Up", "TA Increaser"Sodium bicarbonate (NaHCO₃)Costco baking soda 12 lb$0.50/lb vs $3-5/lb branded
"pH Up"Sodium carbonate (Na₂CO₃) / soda ashArm & Hammer Super Washing Soda$1/lb vs $4/lb branded
"pH Down", "Dry Acid"Sodium bisulfate (NaHSO₄)Pool store / hardware store$2/lb (same product across brands)
"Chlorine Shock", "Pool Shock"Calcium hypochlorite (Cal-hypo) or sodium dichloroisocyanurate (dichlor)For spa: avoid dichlor; use bleach—
"Liquid Chlorine"Sodium hypochlorite (NaOCl) 12.5%Pool store; commercial bleach$3-5/gal
Household bleachSodium hypochlorite 6% or 8.25%Costco / Walmart Clorox$5-7/gal
"Calcium Increaser"Calcium chloride (CaCl₂)Damprid refill, ice melt (food-grade)$1/lb vs $5/lb branded
"Stabilizer", "Conditioner"Cyanuric acidAvoid in spa — do not buy—
"Non-chlorine shock", "MPS"Potassium monopersulfatePool store, same product all brands$4-6/lb
Markup reality Spa store chemicals are typically 3-5× the cost of the same raw chemical bought as a household product. The active ingredient is identical — the only thing you're paying for is the spa-branded label and smaller packaging. The exceptions: bromine tabs, MPS, and Ahh-some have no household equivalent.

Maintenance Cadence