Spas are not small pools. A 400-gallon spa running at 102°F with four people in it is the opposite of a 20,000-gallon pool at 82°F with nobody in it. Higher heat, tiny volume, and 10x the bather-to-water ratio mean chemistry shifts fast and forgiveness is low.
What's different about spa chemistry
- Sanitizer volatilizes faster at 102°F. You lose chlorine (or bromine) quickly.
- Bromine outperforms chlorine in hot water — it stays active longer and doesn't outgas as readily.
- Alkalinity buffering matters more because small pH swings in small volumes are big percentage swings.
- TDS (total dissolved solids) climbs fast, which is why spas need full water changes every 3–4 months.
The 3-month drain rule
There's a rough math for when to drain a spa: spa gallons ÷ 3 ÷ daily bathers = days between refills. A 400-gallon tub used by 2 people daily = 400 ÷ 3 ÷ 2 = ~67 days. Real-world, most residential spas hit the refill point every 3–4 months.