What time should pH be measured in a shrimp pond?

Measure twice: in the morning around 7:00 (lowest pH) and in the afternoon at 15:00–16:00 (highest pH). The difference between the two readings shows whether the pond is stable; the QCVN requires daily fluctuation of no more than 0.5.

If the ammonia test reads 1 mg/L, will the shrimp be affected?

It depends on pH and temperature, because the test kit measures total ammonia; the higher the pH, the larger the toxic NH₃ fraction. Measure pH and temperature at the same time and look up the table.

Why is oxygen high at midday but shrimp still surface in the early morning?

At midday phytoplankton photosynthesise strongly, so oxygen is high; at night phytoplankton and microbes respire and oxygen falls, reaching its lowest at dawn. Only early-morning measurements show whether the pond is short of oxygen.

Can colour-comparison test kits be used?

They can be used to follow trends, but their accuracy is limited. If a result is abnormal, re-measure with a meter or send a sample to a laboratory.

Measuring shrimp pond water: pH, alkalinity, DO, toxins

Measuring at the wrong time, sampling in the wrong place or misreading the result means the numbers are of no help. This article explains how to measure shrimp pond water parameters: when to measure, how to take samples, which thresholds to read against and how to keep a log so you can see trends.

Test kit or meter: which is reliable enough?

According to Professor Claude E. Boyd (Auburn University), shrimp farming only needs pH to ±0.2, total ammonia nitrogen (TAN) to ±0.2 mg/L, oxygen to ±0.5 mg/L and alkalinity to ±5 mg/L to make decisions (Boyd, 2016). pH meters, electronic oxygen meters and small electronic colorimeters can reach this level; drop-count test kits and visual colour-comparison kits struggle to.

He also notes that repeated measurements giving the same number are not necessarily correct: deteriorated reagents or uncalibrated meters give results that are "consistent but wrong". In one survey, a standard alkalinity sample of 80 mg/L was measured by different laboratories at anywhere from 22 to 107 mg/L. How to check:

Periodically test a standard solution (many brands sell them ready-made); if the result is off, review the reagents and procedure.
Oxygen meters: stir clean water vigorously in an open container for about 10 minutes, and the meter should read close to saturation; boiled and cooled water in a tightly sealed bottle should read close to 0 (Boyd, 2016).

Sampling water correctly

The sample must represent the water where the shrimp live and must not change before it is measured (Boyd, 2016):

Sample at the right location and depth. Shrimp live on the bottom, so samples for toxic compounds should be taken near the bottom; H₂S is most concentrated in the water layer just above the bottom (Boyd, 2014).
Sample several points when measuring alkalinity and salinity: combine several samples into one, or measure separately and take the average.
Measure temperature, pH and oxygen on site at the pond, ideally with a probe placed directly in the water.
Measure NH₃ and NO₂ as soon as possible; if the sample must be transported, keep it in an ice box for no more than 4–6 hours.
Salinity and alkalinity change little and can be kept in a tightly sealed bottle and measured within a few days. Rinse equipment with sample water before measuring.

What time of day to measure?

Parameter	When to measure	Reason
Dissolved oxygen	Early morning, just after dawn	Oxygen is lowest in the early morning (Boyd, 2004)
pH, temperature	Morning around 7:00 and afternoon 15:00–16:00	Captures the lowest and highest levels (Kubitza, 2017)
TAN, NO₂	Afternoon, at the same time as pH	High afternoon pH makes NH₃ most toxic
Transparency	Sunny day, sun not covered by cloud	Correct Secchi procedure (Boyd, 2004)
Alkalinity, salinity	Periodically, after heavy rain, after water exchange	Little variation within a day

Kubitza (2017) recommends that ponds fed more than 100 kg of feed/ha/day should measure afternoon pH and ammonia weekly; lined ponds and intensive ponds should measure more often, especially when the weather turns rainy. See also preventing pH and alkalinity shock in the rainy season.

How to read each parameter

pH: look at the range, not just one number

QCVN 02-19:2014/BNNPTNT sets pH at 7–9 with daily fluctuation of no more than 0.5. Kubitza (2017) states that shrimp grow best at pH 7.5–8.5. During the day, phytoplankton photosynthesis raises pH, and at night it falls; the denser the phytoplankton, the stronger the pH swing. A morning–afternoon difference of more than 0.5 (especially 1 unit or more) usually means alkalinity is too low or phytoplankton is too dense, and alkalinity needs to be measured to find the cause. For treatment details, see the article on managing pH and alkalinity.

Alkalinity

The QCVN sets alkalinity at 60–180 mg/L. Alkalinity keeps pH stable. Kubitza (2017) considers that below 30 mg/L liming is required, but that is a general minimum; in practice whiteleg shrimp ponds should be kept at about 120–180 mg/L as CaCO₃ (high-density lined ponds often 150–200 mg/L).

Salinity

The QCVN allows 5–35 ppt. At the same NO₂ level, low-salinity ponds are much more dangerous (see the NO₂ section).

Dissolved oxygen (DO)

The QCVN requires oxygen of 3.5 mg/L or more; in practice keep it at 4 mg/L or above, ideally ≥ 5 mg/L. Only early-morning measurements show the lowest level; midday readings are usually high and can give a false sense of security. Ponds with transparency below 20 cm due to dense phytoplankton often run short of oxygen in the early morning (Boyd, 2004). See also paddlewheel aerator layout and dissolved oxygen.

Ammonia (TAN) and NH₃

Most test kits measure total ammonia nitrogen (TAN), which includes the less toxic NH₄⁺ and the highly toxic NH₃. The proportion of NH₃ rises sharply with pH: in freshwater, at pH 7 only about 0.7% of TAN is NH₃, but at pH 9 it rises to about 42% (Kubitza, 2017). So always measure pH and temperature at the same time as TAN, then look up the table in the article on NH3 and NO2 in lined ponds.

Reference thresholds: the QCVN sets NH₃ below 0.3 mg/L; Kubitza (2026) recommends a threshold of 0.1 mg NH₃/L for marine shrimp.

Nitrite (NO₂)

Nitrite toxicity depends on salinity, because chloride ions reduce nitrite uptake through the gills. Kubitza (2026) compiled data for whiteleg shrimp, taking 10% of the 96-hour median lethal concentration (LC50) as the level of concern:

Salinity	96-hour LC50 (mg N-NO₂/L)	Level of concern (mg N-NO₂/L)
0.6 ppt	5.7	0.6
2 ppt	12.4	1.2
3 ppt	15.2	1.5
15 ppt	76.5	7.6

These are levels of concern, not safe long-term working limits: in the pond, keep NO₂ below 1 mg/L in brackish water and below 0.5 mg/L at low salinity. Nitrite rarely causes problems at salinities above 20 ppt but is common in low-salinity ponds. Note whether the test kit reports NO₂ or N-NO₂; the two numbers are different.

H₂S

The QCVN sets H₂S below 0.05 mg/L; Boyd (2014) advises that brackish-water ponds should not exceed 0.005 mg/L. Test kits usually measure total sulphide; the toxic H₂S fraction decreases as pH rises: at 26°C, the H₂S factor is 0.238 at pH 7.5 but only 0.090 at pH 8.0 (Boyd, 2014). Detectable H₂S usually signals an oxygen-depleted bottom, and aeration needs to be increased. See treating toxic NH3, NO2 and H2S.

Transparency (Secchi disk)

A standard Secchi disk is 20 cm in diameter with 4 alternating black and white quadrants, a line marked in cm and a weight. Measuring procedure according to Boyd (2004):

Measure on a sunny day with the sun not covered by cloud, with your back to the sun.
Keep your eyes 25–50 cm above the water surface, without sunglasses.
Lower the disk slowly until it just disappears and note the depth; raise it until it just reappears and note the depth. Take the average of the two readings.

The QCVN sets transparency at 20–50 cm. Boyd considers 30–45 cm good if the turbidity is due to phytoplankton; below 20 cm is too turbid, above 60 cm too clear. Water made turbid by suspended silt also gives low transparency, so water colour should be checked as well. Water that stays too clear for long periods readily develops benthic algae and lab-lab mats.

Recommended threshold table

Parameter	QCVN 02-19:2014/BNNPTNT	Additional notes
Dissolved oxygen	≥ 3.5 mg/L	Preferably ≥ 4, ideally ≥ 5 mg/L; measure in the early morning
pH	7–9, daily fluctuation ≤ 0.5	Best 7.5–8.5 (Kubitza, 2017)
Alkalinity	60–180 mg/L	Preferably 120–180 mg/L (lined ponds 150–200)
Salinity	5–35 ppt	Low salinity: keep NO₂ lower
Transparency	20–50 cm	30–45 cm is good (Boyd, 2004)
NH₃	< 0.3 mg/L	Marine shrimp preferably < 0.1 mg/L (Kubitza, 2026)
H₂S	< 0.05 mg/L	Preferably ≤ 0.005 mg/L (Boyd, 2014)
Temperature	18–33°C	Best 28–32°C
NO₂	Not specified in the table	Preferably < 1 mg/L (brackish), < 0.5 mg/L (low salinity)

Keeping a pond environment log

A single number means little; the trend over several days is what tells you whether the pond is improving or deteriorating. Boyd (2004) stresses that changes in transparency over time are more important than the value of a single measurement, and advises comparing early-morning oxygen with transparency to find the minimum transparency for each pond.

One page per pond, one line per day: days of culture (DOC), time of measurement, morning and afternoon pH, early-morning oxygen, temperature, transparency, alkalinity, salinity, TAN, NO₂, H₂S, feed quantity, weather, products and doses used. Mark days of rain and water exchange. Cross-check against the care schedule by days of culture to see which stages need more frequent measurement.

Supporting Noben products

When results exceed the thresholds, use according to the label:

SUPER OXY 02: supplies oxygen quickly; emergency use when shrimp surface due to oxygen shortage. 2–3 kg/1,000m³, spread evenly over the pond surface, together with running the aerators.
AQUA YUCCA: rapidly absorbs toxic compounds. Routine 500g/5,000m³ every 7 days; when toxic compounds are high 500g/3,000m³, applied 2–3 times in a row.
NB BROWN: quickly builds water colour when transparency is too high. Sudden loss of phytoplankton: 250g/1,000m³.
PROTICBEST: probiotic that cleans the pond bottom and breaks down organic waste and toxic compounds at the pond bottom. Apply directly 1 litre/3,000m³, routinely every 3–5 days.

Use the dose calculator to convert doses to the pond volume. See also the solutions for oxygen shortage and stress shock and toxic gases at the pond bottom and liner slime.

How to measure and read shrimp pond water parameters: pH, alkalinity, oxygen, NH3, NO2, H2S, transparency