Managing pH and alkalinity in shrimp ponds: standard ranges and how to adjust them
pH and alkalinity govern moulting, shell hardening and gut health. Ranges for whiteleg shrimp, raising alkalinity, lowering pH and fixing post-rain swings.
In this article
pH and alkalinity are two parameters farmers need to measure every day. Stable pH keeps shrimp healthy and feeding well; sufficient alkalinity helps shrimp moult and harden their shells quickly, while keeping pH from fluctuating strongly.
Standard ranges for whiteleg shrimp
| Parameter | Suitable range |
|---|---|
| pH | 7.5–8.5 |
| Morning–afternoon pH fluctuation | no more than 0.5 |
| Alkalinity (brackish-water ponds) | 120–180 mg CaCO₃/L |
| Alkalinity (low-salinity ponds) | 150 mg CaCO₃/L or more |
Measure pH twice a day, at around 6:00 in the morning and 14:00 in the afternoon. Measure alkalinity 2–3 times a week.
Why does pH fluctuate strongly?
- Phytoplankton too dense: during the day phytoplankton absorb CO₂, pushing pH up; at night they respire, pulling pH down.
- Low alkalinity: the water has no buffering capacity, so pH rises and falls erratically.
- Heavy rain: rainwater is acidic and washes acid sulphate (alum) from the pond banks, making pH drop quickly.
- Acid sulphate soil: ponds in acid sulphate areas often have low and unstable pH.
How to raise alkalinity
- Apply lime (CaCO₃) or dolomite at night, split into several applications. At pH 8 or above CaCO₃ dissolves poorly; to raise alkalinity quickly without pushing pH up, use sodium bicarbonate (NaHCO₃).
- Use alkalinity-raising minerals such as MIRAL N8, PREMIX 10000 or RIMIX for lined ponds, to raise alkalinity and supply shell-hardening minerals at the same time.
- Do not raise alkalinity too quickly; an increase of about 10–20 mg/L per day is safe.
How to deal with high afternoon pH
- Reduce phytoplankton density with algae-controlling probiotics and partial water exchange.
- Apply molasses or granulated sugar in the morning at a suitable dose for bacteria to use, which helps lower pH. Molasses increases bacterial oxygen demand, so run enough aerators and monitor DO that night.
- Do not use quicklime (CaO) when pH is already high.
How to deal with low pH and acid sulphate
Frequently asked questions
Is an afternoon pH of 9 dangerous?
Yes. pH above 9 makes NH₃ very toxic and shrimp are easily shocked. Reduce phytoplankton density and bring pH back below 8.5.
Why is alkalinity still low after liming?
Possible causes are an insufficient amount of lime, pond leakage, dilution by heavy rain, or mass moulting consuming large amounts of minerals. Farmers should split the dose into smaller applications and re-measure after 1–2 days.
When should lime be applied?
Apply lime in the evening, at around 21:00–22:00, to avoid pushing pH up in the afternoon and to help the shrimp moult smoothly.
