Quick answer
Test pH and total alkalinity together. Aim for pH 7.2–7.6 and alkalinity around 80–120 ppm, then use a dose calculator or the product label based on your pool volume. Circulate for at least 30 minutes, retest after a few hours and never mix pH products with chlorine.
What you're seeing
- High pH: cloudy water, white scale at the waterline or weak chlorine.
- Low pH: corroded metal, pitted fittings, stinging eyes or a rough liner.
- pH readings swing widely from one test to the next.
- Chlorine seems ineffective even when the free chlorine reading looks reasonable.
What usually causes it
Cause 01
Total alkalinity is the underlying problem
Alkalinity buffers pH. If it is too low, pH bounces; if it is too high, pH keeps climbing and resists acid.
Cause 02
Source water and aeration
GTA fill water can be alkaline, and returns aimed at the surface or a running water feature drive pH up through aeration.
Cause 03
Chemical overdose
Too much soda ash, pH increaser or chlorinating product can push pH high. Adding acid and chlorine at the same time is also unsafe.
Cause 04
Rain, runoff and heavy use
Rain and splash-out dilute alkalinity, while body oils and sunscreen change demand. A pool can drift after a storm or a busy weekend.
How to fix it, in order
Work through these steps once, with the pump running and the filter clean. Do not stack a second chemical dose before the first has circulated and you have retested.
- 01
Test pH and alkalinity together
Record both before changing anything. A pH correction based on a single strip result often overshoots.
- 02
Fix alkalinity first if it is far out
If alkalinity is below range, raise it and let it circulate before adjusting pH. If it is very high, a professional may recommend a controlled acid addition.
- 03
Dose for your volume
Use a pool-volume calculator and the product label. Add acid to water, never water to acid, and keep chlorine products separate.
- 04
Circulate and retest
Run the pump for at least 30 minutes and retest after a few hours. Do not stack another full dose because the first one has not had time to mix.
- 05
Watch the trend, not one reading
Log pH and alkalinity for a week. A pool that keeps rising needs a different routine, not a bigger bottle of pH decreaser.
What not to do
- Never mix pH adjuster with chlorine or add them at the same time.
- Do not swim when pH is far outside the normal range or the water is irritating skin and eyes.
- Do not use a metal brush on a vinyl liner to remove scale; use a soft brush and correct the chemistry.
Keeping it from coming back
- Test pH and alkalinity weekly, and after heavy rain or a crowd.
- Aim returns slightly downward so the surface is not constantly aerated.
- Keep a written log; trends are more useful than a single test.
- Check fill-water chemistry when topping up after a leak or opening.
When to stop and call a pro
Do not force it
Call a professional if pH will not hold after two properly spaced adjustments, if alkalinity is very high or very low, or if you see scale, corrosion or liner damage. Those are signs of a longer-term imbalance, not a one-dose problem.
Compare local repair helpGeneral guidance for residential pools in Ontario. Chemical doses depend on pool volume and test results; follow the product label and keep gas, electrical and major shell work with licensed trades.