If you keep adding soda ash and pH keeps dropping, you are treating the symptom, not the cause. The real question is: why is your pool losing the ability to hold a stable pH? The answer is almost always alkalinity.
Total alkalinity (TA) is the carbonate buffer that stabilizes pH. Below 60 ppm, pH has almost no resistance to change — it swings with every chemical addition, rain drop, or CO2 change. This is called pH bounce.
Fix: Test alkalinity. If below 80 ppm, raise it to 90–110 ppm with sodium bicarbonate. Stable alkalinity = stable pH.
Trichlor tablets have a pH of approximately 2.8–3.0, making them extremely acidic. Every tablet that dissolves pushes pool pH down. Pools maintained exclusively with trichlor tablets commonly need soda ash added every few weeks to counteract the pH drop.
Fix: Reduce tablet use and supplement with liquid chlorine (which raises pH slightly). Or accept that you will add soda ash periodically.
Rainwater typically has a pH of 5.5–6.0 (and lower in polluted areas). A significant rain event adds large amounts of acidic water, diluting the pool's alkalinity buffer and dropping pH. The effect is more pronounced in smaller pools.
Fix: Test after every significant rain event and re-balance chemistry before swimming.
Swimmers introduce carbon dioxide (from breathing near the water surface), sweat, and urine — all of which are acidic. A pool party with 15 swimmers can drop pH by 0.3–0.5 in an afternoon.
Actively growing algae produces CO2 during cellular respiration. In an algae-infested pool, CO2 dissolves in the water as carbonic acid, lowering pH. If pH keeps dropping despite correct alkalinity, check for early algae.
If your tap or well water has a naturally low pH (below 7.0), topping off the pool adds acidic water. Test your source water pH and factor it into your management routine.
The correct order of operations: always adjust alkalinity first, then adjust pH. If alkalinity is correct, pH is much more stable and requires less frequent correction.
| Clue | Likely Cause |
|---|---|
| pH drops fast regardless of weather | Low alkalinity (below 60 ppm) |
| pH drops after rain | Rain dilution + low alkalinity |
| pH drops mainly on swim days | Bather load + CO2 |
| pH drops steadily throughout season | Trichlor tablet accumulation |
| pH drops with green or hazy water | Algae CO2 production |
PoolLens plots your pH readings over days and weeks. Identifying whether pH drops after rain, after swim days, or continuously reveals the root cause — not just the symptom.
Open PoolLens Free →Add sodium carbonate (soda ash) — about 6 oz per 10,000 gallons raises pH by 0.2. Broadcast over the deep end with the pump running. But fix alkalinity first — without adequate alkalinity, pH will drop again quickly.
Yes. Low total alkalinity (below 60–70 ppm) removes the carbonate buffer that stabilizes pH. Without this buffer, pH swings wildly. Fix alkalinity first (raise to 80–120 ppm with sodium bicarbonate), then adjust pH.
Yes. Trichlor tablets have a pH of about 2.8–3.0, which is very acidic. Regular use steadily pulls pool pH down. Pools maintained mainly with trichlor tablets often need periodic soda ash additions. Switching partially to liquid chlorine reduces this effect.
Rainwater is mildly acidic (pH 5.5–6.0). Heavy rain adds significant acidic water to the pool, diluting the alkalinity buffer and pulling pH down. After any significant rain, test and re-balance pH and alkalinity before swimming.
Yes. Below 7.0, pool water becomes corrosive — etching plaster, corroding metal fittings, and degrading heater components. Chlorine burns off extremely rapidly at low pH. Bring pH back above 7.2 as quickly as possible with soda ash.