Why It Matters — and How to Control It
Dissolved oxygen (DO) is one of the most critical — and most misunderstood — parameters in aquaculture and aquarium systems.
Fish, crustaceans, and aerobic microorganisms rely on oxygen dissolved in water to survive. When oxygen levels fall, stress rises rapidly, growth slows, and mortality can occur with little warning.
Understanding how dissolved oxygen behaves — and measuring it correctly — is essential to maintaining healthy aquatic systems.
According to the Global Seafood Alliance, dissolved oxygen is a major concern in aquaculture, as low oxygen levels can quickly stress or kill aquatic species.
What Is Dissolved Oxygen?
Dissolved oxygen refers to the small amount of oxygen gas (O₂) that is held in water.
Unlike air, water can only hold a limited amount of oxygen, and that capacity changes constantly depending on environmental conditions. In managed systems such as tanks, ponds, raceways, and aquariums, dissolved oxygen must be actively maintained.
DO is typically measured in milligrams per litre (mg/L).
Safe Dissolved Oxygen Levels
In most aquaculture and aquarium applications:
- 5–14 mg/L is generally considered healthy
- Below ~5 mg/L, many fish species experience stress
- Below ~3 mg/L, serious risk of mortality occurs
- Some species require higher minimum levels
For example:
- Cold-water species such as trout require higher DO levels
- Warm-water species may tolerate lower levels — but only briefly
The key point is not a single number, but consistency and trend.
Why Dissolved Oxygen Changes
Temperature
As water temperature increases, its ability to hold oxygen decreases.
This makes warm weather one of the highest-risk periods for:
- Aquaculture ponds
- Outdoor tanks
- Heavily stocked systems
Atmospheric Pressure
Lower pressure means less oxygen can remain dissolved in water. While this is more pronounced at altitude, it also affects systems during weather changes.
Water Movement and Aeration
Oxygen enters water through:
- Surface diffusion
- Aeration
- Turbulence and mixing
In natural systems this occurs via flow over rocks.
In managed systems, it relies entirely on mechanical aeration and circulation.
Oxygen Replenishment vs Oxygen Consumption
Oxygen is added to water through:
- Aerators and diffusers
- Water movement and surface agitation
- Photosynthesis (during daylight hours)
Oxygen is consumed by:
- Fish and aquatic animals
- Microbial activity
- Decomposition of waste
- Uneaten feed
- Biofilters
In heavily stocked systems, oxygen demand can rise very quickly.
The Night-Time Dissolved Oxygen Trap
One of the most common causes of unexpected fish loss occurs overnight.
At night:
- Photosynthesis stops
- Respiration continues
- Oxygen consumption exceeds replenishment
DO levels often reach their lowest point just before dawn — when systems are rarely checked.
This is why systems that appear “fine during the day” can fail overnight.
Aerobic vs Anaerobic Conditions
When dissolved oxygen remains adequate:
- Aerobic microorganisms dominate
- Organic waste is oxidised efficiently
- Water remains stable and low-odour
When DO drops too low:
- Anaerobic organisms take over
- Putrefactive decomposition occurs
- Harmful compounds such as sulphides and amines form
- Water quality degrades rapidly
Maintaining oxygenation protects not only fish, but the entire biological balance of the system.
Why Spot Checks Are Often Not Enough
Manual DO testing provides a snapshot, not the full picture.
In dynamic systems:
- Oxygen can change hour-to-hour
- A single daily reading may miss critical lows
- Problems are often detected after damage has occurred
Professional operators increasingly rely on:
- Regular spot checks for verification
- Continuous monitoring for early warning
Measuring Dissolved Oxygen with Confidence
Accurate DO measurement allows operators to:
- Detect downward trends early
- Adjust aeration before stress occurs
- Protect stock during high-risk periods
- Make informed operational decisions
Depending on the application, this may involve:
- Portable DO meters for spot testing
- Fixed or submersible sensors for continuous monitoring
- Integration with alarms or online dashboards
Measure With Confidence
Dissolved oxygen is not a “set and forget” parameter.
It responds continuously to:
- Temperature
- Stocking density
- Feeding
- Biological activity
- Time of day
The only way to manage it reliably is to measure it properly and monitor trends, not just numbers.
That’s how professionals measure with confidence.
Call to Action
Protect aquatic life by measuring dissolved oxygen accurately
Meascom offers portable dissolved oxygen meters for spot testing, as well as submersible sensors and monitoring solutions for continuous DO measurement in aquaculture and aquarium systems.
