Getting pH and alkalinity right is the most important skill in hot tub water care. These two parameters interact with each other and affect everything else — sanitizer effectiveness, equipment longevity, and bather comfort. Once you understand how they work and how to adjust them correctly, managing your water becomes straightforward.
TL;DR
- Adjust total alkalinity before adjusting pH — alkalinity stabilizes pH
- Raise alkalinity with sodium bicarbonate (baking soda)
- Lower alkalinity with muriatic acid or pH decreaser
- Raise pH with sodium carbonate (pH up) or aeration
- Lower pH with muriatic acid or pH decreaser
Why Order of Adjustment Matters
Total alkalinity and pH are chemically linked — changing one affects the other. The correct protocol is to adjust alkalinity first, then pH. When alkalinity is in the correct range, pH adjustments become more stable and predictable.
If you adjust pH first and then correct alkalinity, you will find that the alkalinity adjustment shifts the pH again, requiring another round of correction. Starting with alkalinity eliminates this cycle.
Understanding Your Target Ranges
Total alkalinity target: 80 to 120 ppm. Lower than 80 ppm and pH will fluctuate unpredictably. Higher than 120 ppm and pH will resist adjustment and tend to drift high.
pH target: 7.2 to 7.8. Below 7.0 is corrosive and irritating. Above 8.0, sanitizer becomes significantly less effective and scale begins to form.
Always test both before adjusting either. You need to know where both parameters currently sit to plan your adjustments correctly.
How to Raise Alkalinity
Sodium bicarbonate (baking soda) raises total alkalinity without significantly affecting pH. It is inexpensive, widely available, and safe to handle.
The general guideline is approximately 1.5 tablespoons of sodium bicarbonate per 1,000 liters to raise alkalinity by 10 ppm. Add it directly to the spa water with jets running, wait 30 minutes for full circulation, and test again. Make incremental adjustments rather than adding the full calculated dose at once.
How to Lower Alkalinity
Muriatic acid (hydrochloric acid) or a commercially formulated pH decreaser lowers both alkalinity and pH. When lowering alkalinity, the technique matters: add acid directly to a concentrated point in the water (rather than distributing it widely) with jets turned off, which preferentially lowers alkalinity over pH. Allow the acid to mix over 30 minutes before turning the jets back on and testing.
Acid is hazardous — handle with appropriate care, never add water to acid, and add acid to water rather than the reverse. Follow all product safety instructions.
How to Raise pH
Sodium carbonate (soda ash, sold as pH Increaser) raises pH. Add it directly to the spa with the jets running. It raises pH relatively quickly and has a smaller effect on alkalinity than sodium bicarbonate.
Aeration — running the jets with the air injectors open — also raises pH by off-gassing carbon dioxide from the water. In some situations, running the spa open for 30 to 60 minutes can raise pH without adding chemicals. This is useful when pH is marginally low and alkalinity is already correct.
How to Lower pH
Muriatic acid or commercially formulated pH decreaser lowers pH. With jets running, add the product to the water away from the skimmer. The adjustment takes 30 minutes to circulate fully.
pH decreaser and muriatic acid are functionally similar — both lower pH and alkalinity. Muriatic acid is less expensive but more hazardous to handle. pH decreaser products are safer to handle but cost more per unit of adjustment.
Making Small Adjustments
The most common mistake in water balancing is over-correcting. Add the minimum calculated dose, wait for full circulation, test, and adjust again if needed. It is much easier to make two small adjustments than to fix an over-correction.
When correcting both alkalinity and pH, wait at least 30 minutes between adjustments to allow each change to stabilize before assessing where the levels now sit.
When Nothing Seems to Work
If pH or alkalinity will not respond to appropriate adjustments, the likely cause is either very high TDS (which buffers chemical reactions) or a water source issue. Test TDS if you have access to a meter. If TDS is very high, a water change is the most effective solution — fresh water with normal mineral content responds predictably to standard adjustment products.
Bring a water sample to a Poolboy showroom for free testing if you are struggling to get chemistry under control.
New Brunswick Perspective
The most useful perspective on hot tub water chemistry: small adjustments, made regularly, are always easier than large corrections made infrequently. If you test every week and make a small alkalinity or pH correction when needed, you will never find yourself dealing with chemistry that has drifted far out of range. The goal is to keep the water within the target ranges consistently, not to chase exact ideal numbers with every test.
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