Why Dissolved Oxygen Drops at Night in Aquaculture Ponds Malaysia

Dissolved oxygen in an aquaculture pond can be acceptable during the afternoon but fall significantly during the night. This daily change occurs because oxygen production decreases after sunset while fish, shrimp, plants and microorganisms continue consuming oxygen.

Using an Azovtes dissolved oxygen meter for evening and early-morning monitoring helps aquaculture operators identify overnight oxygen risk before fish or shrimp show severe distress.

Why Is Oxygen Higher During the Day?

During daylight, algae and aquatic plants use sunlight for photosynthesis. This process can release oxygen into the water.

Daytime dissolved oxygen may increase because of:

Strong sunlight
Active photosynthesis
Wind movement
Mechanical aeration
Incoming water
Surface-water mixing

A high afternoon result may therefore reflect daytime oxygen production rather than the pond’s lowest daily condition.

An afternoon reading alone cannot confirm that oxygen will remain adequate throughout the night.

What Changes After Sunset?

When sunlight disappears, photosynthesis stops. However, oxygen consumption continues.

Oxygen is still used by:

Fish
Shrimp
Algae
Aquatic plants
Bacteria
Microorganisms
Decomposing organic matter
Pond-bottom biological activity

As the night continues, dissolved oxygen may decline until sunlight returns and photosynthesis resumes.

When Is Dissolved Oxygen Usually Lowest?

The lowest dissolved oxygen level commonly occurs around the period before sunrise.

At this time:

Photosynthesis has been absent for several hours
Fish and microorganisms have continued respiring
Organic matter has continued decomposing
Aeration may have been insufficient
The pond has reached the end of its overnight oxygen demand

Pre-sunrise measurement is one of the most valuable checks for identifying overnight oxygen risk.

The exact lowest time varies according to the pond, weather, season and aeration schedule.

Major Causes of Night-Time Oxygen Depletion

High Stocking Density

More fish or shrimp means greater total oxygen demand.

As biomass increases during the production cycle, an aeration system that was adequate earlier may become insufficient.

Excess Feed

Uneaten feed and waste contribute to organic loading. Microorganisms consume oxygen while breaking down this material.

Overfeeding may therefore increase overnight oxygen demand even when the pond looks normal during the day.

Algae and Phytoplankton

Algae can produce oxygen during daylight, but they also consume oxygen through respiration at night.

A dense algae bloom may produce very high afternoon DO followed by a much lower pre-dawn result.

A high daytime reading in an algae-rich pond may hide a large daily oxygen fluctuation.

Decomposing Organic Matter

Dead algae, feed, waste and plant material are decomposed by microorganisms. This biological activity consumes oxygen.

Organic material may accumulate in:

Pond corners
Feeding zones
Areas with weak circulation
Bottom sediment
Dead zones away from aerators

Hot Water

Warmer water generally holds less dissolved oxygen. At the same time, fish and microorganisms may consume oxygen more rapidly under warmer conditions.

Hot Malaysian weather can therefore create a combination of lower oxygen capacity and higher biological demand.

Cloudy or Rainy Weather

Several cloudy days can reduce photosynthesis and daytime oxygen production.

Heavy rain may also:

Reduce sunlight
Change water temperature
Disturb pond layers
Introduce organic runoff
Affect pH
Alter algae conditions

Ponds should receive additional attention during extended cloudy or rainy periods.

Algae Die-Off

When algae die suddenly, photosynthetic oxygen production decreases while decomposition increases oxygen consumption.

Possible warning signs include:

Sudden water-colour change
Floating dead algae
Unusual odour
Reduced water clarity
Rapid DO decline
Changes after chemical treatment

Algae die-off can create a serious oxygen demand.

Insufficient Aeration

Aerators may be:

Undersized
Poorly positioned
Operated for too few hours
Mechanically damaged
Blocked
Affected by power failure
Unable to circulate the complete pond

Testing only near the aerator may fail to reveal low-oxygen zones elsewhere.

Warning Signs of Low Pond Oxygen

Fish or shrimp may show:

Gasping at the surface
Gathering near water inlets
Crowding around aerators
Reduced feeding
Slow movement
Unusual swimming
Sudden stress
Mortality in severe cases

Visible distress is a late warning sign. Routine DO measurement should identify declining oxygen before this stage.

Recommended Monitoring Times

A practical monitoring programme may include:

Late afternoon
Evening
Midnight during high-risk conditions
Before sunrise
After sunrise
After heavy rain
During prolonged cloudy weather
After heavy feeding
After an aerator problem
During algae changes

Comparing afternoon and pre-dawn readings reveals the size of the overnight oxygen drop.

Where Should Night-Time DO Be Measured?

Measure at consistent locations such as:

Near the aerator
Away from the aerator
Pond centre
Feeding area
Water inlet
Water outlet
Weak-circulation area
Different depths where necessary

A satisfactory reading beside an aerator does not prove that distant pond areas are equally oxygenated.

How Temperature Affects the Reading

Water temperature and dissolved oxygen should be recorded together.

As temperature changes:

Oxygen solubility changes
Fish metabolism may change
Microbial activity may change
Sensor response may change
Pond stratification may develop

A DO result without its temperature and measurement time provides incomplete information.

Using an Azovtes Dissolved Oxygen Meter

Azovtes offers membrane-based and fluorescence-based dissolved oxygen meters for portable water testing.

For night and early-morning pond monitoring, consider:

Display readability
Probe cable length
Battery condition
Sensor response
Required sample movement
Ease of cleaning
Calibration procedure
Replacement consumables

A fluorescence meter may reduce certain membrane-related maintenance tasks, while a membrane meter may provide a more economical entry option for trained users.

Night-Time Measurement Procedure

Inspect and calibrate the meter before fieldwork.
Use established pond monitoring points.
Lower the probe to the required depth.
Prevent contact with bottom sediment.
Remove bubbles from the sensing area.
Provide required probe movement.
Allow the reading and temperature to stabilise.
Record the exact time.
Note which aerators are operating.
Repeat at other locations.
Rinse the probe between ponds.
Store the instrument correctly after use.

Use adequate lighting and follow safe night-working procedures around ponds.

Common Monitoring Mistakes

Avoid:

Measuring only during sunny afternoons
Waiting until fish gasp at the surface
Testing only beside the aerator
Using inconsistent depths
Ignoring temperature
Recording before stabilisation
Using a dirty or damaged probe
Failing to note aerator status
Comparing results from different times without context
Assuming all ponds behave identically

What Should You Do When DO Is Falling?

When a low or falling reading is detected:

Verify the measurement.
Inspect the probe for bubbles or contamination.
Repeat at another pond location.
Check aerators and power supply.
Observe fish or shrimp behaviour.
Review recent feeding and weather.
Follow the farm’s approved low-oxygen response plan.
Continue measuring until the condition stabilises.

Do not delay an established emergency response while repeatedly troubleshooting the meter.

Building an Overnight Oxygen Profile

For a more complete assessment, record DO at several times:

Monitoring Time	Purpose
Late afternoon	Observe daytime oxygen peak
After sunset	Identify the start of decline
Midnight	Assess overnight trend
Before sunrise	Identify likely daily minimum
After sunrise	Observe recovery

Repeat this profile during normal and high-risk weather to understand the pond’s typical behaviour.

Reducing Night-Time Oxygen Risk

Depending on the farm’s approved management programme, preventive actions may include:

Maintaining aerators
Improving aerator placement
Reviewing operating hours
Controlling feed
Managing stocking density
Removing excess organic waste
Monitoring algae
Improving water circulation
Preparing backup power
Increasing measurement frequency during risky weather

Water-quality decisions should be based on verified measurements and professional aquaculture practices.

Azovtes Aquaculture DO Meter Supplier Malaysia

MTM Precision supplies portable dissolved oxygen and water-quality meters for fish farms, shrimp farms, hatcheries and aquaculture projects across Malaysia.

Coverage includes Selangor, Kuala Lumpur, Johor, Penang, Perak, Melaka, Negeri Sembilan, Pahang, Kedah, Perlis, Terengganu, Kelantan, Sabah and Sarawak.

Contact MTM Precision

MTM Precision Sdn Bhd
Website: www.mtmpre.com.my
Email: mtmpre@yahoo.com
WhatsApp: 016-660 7346

Showroom & Service Centre

No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia

Contact MTM Precision for assistance selecting an Azovtes dissolved oxygen meter for day and night aquaculture monitoring in Malaysia.