Last updated: 2 June 2026
Spa and hot tub water chemistry isn’t pool chemistry on a smaller scale. The variables that make a pool simple, large volume, low temperature, slow chemical turnover, flip in a spa. Smaller volume, higher temperature, and a 5-minute turnover rate mean every chemistry mistake amplifies fast. A pH or chlorine excursion that would take a pool 24 hours to expose can show up in a spa in 20 minutes as a cloudy, foaming, or eye-stinging surprise.
The good news: spa chemistry is also faster to correct. The small volume means dosing changes settle in minutes rather than hours, and a full drain-and-refill, the nuclear option in spa maintenance, costs a few hundred litres of water and an evening.
What makes spa chemistry different
Three variables fundamentally change the chemistry approach compared to a pool:
- Temperature. Spas run at 37–40 °C. Chlorine dissipates 2–3× faster at spa temperatures than at pool temperatures. Bromine, the most common alternative sanitiser, doesn’t have this temperature-dependent decay problem, which is why a lot of spas run bromine instead of chlorine.
- Bather load per volume. A 1,500 L spa with four people in it has a bather-to-water ratio equivalent to a 50,000 L pool with 130 swimmers. The contaminant load per litre is enormous, which means chlorine demand is enormous.
- Aeration. The jets that make spas spas also drive off-gas CO₂, which steadily raises pH. A spa run hard will drift from pH 7.4 to 7.8+ in a week without intervention.
Bromine vs chlorine for spas, which to choose
The classic spa-owner question. Both work. Here’s the comparison most pool shop staff will give you in 30 seconds, expanded with the parts they leave out:
| Factor | Bromine | Chlorine |
|---|---|---|
| Stability at spa temperature | Excellent, bromine doesn’t dissipate the same way chlorine does at 38 °C+. | Faster decay. Acceptable but needs more frequent additions. |
| Effectiveness at high pH | Less pH-sensitive. Works at 7.6–8.2 nearly as well as 7.4. | Strongly pH-sensitive. At pH 8.0, only ~30% of FC is active. |
| Skin/eye irritation | Generally gentler. Some users with sensitive skin still react. | More irritating, especially as chloramines. |
| Cost | $25–35/kg for bromine granules or tablets. | $8–15/kg for dichlor granular. |
| Smell | Mild bromamine smell at saturation, less recognisable than chlorine. | Classic “pool smell” when chloramines build. |
| Drain frequency | Same, every 3–4 months regardless. | Same. |
| Reactivates with shock? | Yes, used bromide can be re-oxidised with non-chlorine shock, extending its life cycle. | No, used chlorine is gone. |
The honest summary: bromine is the better choice for most residential spas, especially indoor or covered spas where the cost difference is offset by less frequent dosing. Chlorine works fine if you’re disciplined about pH and willing to dose more often.
Spa-specific target parameters
| Parameter | Target range | Notes |
|---|---|---|
| Free Chlorine | 3–5 ppm | Higher than a pool because of higher bather load per litre. |
| Bromine (if bromine system) | 3–6 ppm | Same effective sanitisation as 3–5 ppm chlorine. |
| pH | 7.2–7.6 | Tighter range than a pool. Spa pH drifts up fast from aeration. |
| Total Alkalinity | 80–100 ppm | Lower end of pool range, tighter buffer is desirable. |
| Calcium Hardness | 150–250 ppm | Lower than pools. Higher CH causes scale on heating elements. |
| Cyanuric Acid (chlorine spas only) | 0–30 ppm (indoor: 0; outdoor: up to 30) | Indoor spas don’t need CYA, there’s no UV. |
| Total Dissolved Solids (TDS) | < 1,500 ppm above fill water | When TDS climbs > 1,500 ppm above the original fill water, drain and refill. |
The single biggest spa-vs-pool difference: The drain cycle. Spas need to be fully drained and refilled every 3–4 months. There’s no way around this. The combination of high contaminant concentration, high TDS build-up, and elevated calcium hardness eventually pushes the water beyond chemistry’s ability to correct it. Reset by draining.
The drain-and-refill cycle
The math is fairly simple: take your spa’s volume in litres and divide by 12 × bather count. The result is the number of days between drains. A 1,500 L spa with 3 regular bathers should drain every 1,500 ÷ 36 = ~42 days. A 1,500 L spa with 6 bathers should drain every 1,500 ÷ 72 = ~21 days.
Most residential spa owners drain quarterly (every 90 days) regardless. That’s fine for light use (1–2 bathers a few times a week) but inadequate for heavy use (4+ bathers daily). If your spa starts foaming despite chemistry being in spec, your TDS is probably above 1,500 above fill, drain.
Common spa problems and their fixes
Foaming
Caused by body oils, lotions, sunscreen, soap residues, and surfactants accumulating in the water. The standard “anti-foam” products mask the symptom but don’t fix the cause. Real fixes, in order:
- Insist on a rinse-off before everyone enters.
- Clean the filter, most spa foam comes from dirty filter cartridges releasing oils back into the water.
- Drain and refill if TDS is high.
Cloudy water
Usually one of three things: low sanitiser (chloramines/bromamines), high pH (precipitated calcium), or filter failure. Test FC/bromine, test pH, then inspect the filter cartridge. If all three are fine, you’re looking at fine particulate that needs a flocculant or a 24-hour filter run.
Hot tub rash (folliculitis)
Caused by Pseudomonas aeruginosa bacteria thriving in under-sanitised hot water. Almost always indicates the spa wasn’t properly sanitised during use. Symptoms appear 24–72 hours after bathing. Prevention: maintain FC at 3–5 ppm minimum, shock weekly, drain on schedule.
If multiple bathers develop hot tub rash: immediately drain the spa, sanitise the cabinet and filter housing with diluted bleach, replace the filter cartridge, refill, and bring sanitiser back to spec before anyone uses it. Don’t try to fix this through chemistry alone, biofilm in the plumbing needs physical eradication.
A real-world worked example: post-party spa recovery
1,500 L outdoor spa, six bathers Saturday evening. Sunday morning tests:
- FC: 0 ppm (gone)
- Combined Chlorine: 2 ppm (significant chloramines)
- pH: 8.0 (drifted high)
- TA: 90 ppm ✓
- Foam visible at jet outlets
Recovery sequence:
- Lower pH first: 30 mL of dry acid (sodium bisulfate) brings pH from 8.0 → 7.4. Wait 30 minutes, re-test.
- Shock to break combined chlorine: need FC at roughly 20 ppm (10× the CC of 2 ppm). That’s 20 g of dichlor granular for a 1,500 L spa. Add through the skimmer, jets running, lid open for off-gassing.
- Run jets and pump for 60 minutes minimum. Re-test FC, should be back to a normal 3–5 ppm range within 4–6 hours.
- Pull and rinse the filter cartridge to clear the foaming agents.
- If TDS is the cause (which it likely is after six bathers), schedule a drain and refill within 2 weeks.
Sources & references
- Trouble Free Pool, ABCs of Pool Water Chemistry (community-developed reference based on the Pickens method)
- ANSI/APSP-11, Water Quality Standards in Public Pools (Association of Pool & Spa Professionals)
- CDC, Operating Public Hot Tubs (US Centers for Disease Control)
- WHO Guidelines for safe recreational water environments, Vol 2: Swimming Pools and Similar Environments
Frequently asked questions
How often should I drain and refill my spa?
Every 3–4 months for typical residential use. Heavy use (4+ bathers daily) needs draining every 3–6 weeks. The math is: spa volume in litres divided by 12 × number of regular bathers equals days between drains. A 1,500 L spa with 3 regular bathers should drain every ~42 days.
Bromine or chlorine, which is better for spas?
Bromine for most residential spas, especially indoor or covered ones. It’s more stable at spa temperatures, less pH-sensitive, and can be reactivated with non-chlorine shock. Chlorine is cheaper per dose but needs more frequent attention and is more irritating at spa temperatures.
Why does my spa foam?
Foam comes from body oils, lotions, sunscreen, soap residues, and surfactants in the water. The fixes are: rinse off before bathing, clean the filter cartridge regularly, and drain when TDS climbs. Anti-foam products mask the symptom but don’t address the cause.
What is hot tub rash?
Hot tub rash (folliculitis) is caused by Pseudomonas aeruginosa bacteria in inadequately sanitised hot water. Symptoms appear 24–72 hours after bathing, itchy red bumps usually on the torso. Prevention is keeping free chlorine or bromine at the right level constantly and shocking weekly.
Do indoor spas need cyanuric acid (stabiliser)?
No. Cyanuric acid stabilises chlorine against UV. Indoor spas have no UV exposure, so CYA serves no purpose and just contributes to chlorine lock over time. Keep CYA at zero in indoor spas. Outdoor spas can tolerate 20–30 ppm of CYA for partial UV protection.
How do I get rid of the smell in my spa?
Spa smell almost always indicates chloramines or bromamines, used-up sanitiser combined with organic matter. The fix is shocking the spa to break-point chlorination, not adding more chlorine. Once shocked, leave the cover off for 30 minutes to let the chloramines off-gas.
Will the Water Test Calculator app work for my spa?
Yes. The app has a dedicated spa profile that handles the tighter pH range, higher chlorine targets, lower CYA recommendations, and different shock math compared to a swimming pool. Launching on iOS and Android in approximately 10 days.
Hot tub & spa water testing, simplified
Spas need higher chlorine, tighter pH, and faster response than pools. Water Test Calculator ships with a dedicated spa profile that handles all of it. Free checklist + launch alert below.
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