In fish farming, dissolved oxygen is the factor that causes damage the fastest. An incorrect pH level stresses the fish for days, while a drop in oxygen levels can happen overnight and affect an entire stock. Nevertheless, measurements are often taken only after the fish show visible signs of distress—and even then, only at the spot that’s easiest to reach. This article explains what levels you should aim for, when and where measurements are truly meaningful, and which sensor is best suited for which operation.
If you’re looking for an overview of all water parameters, start on our aquaculture page. This article focuses solely on oxygen.
Guidelines for Stocking
The amount of oxygen required depends primarily on the fish species, temperature, stocking density, and feeding regimen. The data in the technical literature therefore varies; the following table provides guidelines rather than strict limits.
| Fish Group | Aim | Critical |
|---|---|---|
| Cold-water fish such as trout and char | at least 6 mg/l, preferably higher | less than 4 mg/l |
| Warm-water fish such as carp | At least 5 mg/l | less than 2 mg/l; growth is already impaired at levels below 3 mg/l |
“Critical” does not mean that everything is fine. Long before an acute deficiency sets in, the fish eat less, utilize the feed less efficiently, and grow more slowly. Chronically low levels therefore cost money even if not a single fish dies.
mg/l or percent: why you need both
Most meters display oxygen levels in mg/l and as a percentage of saturation. For the fish, mg/l is what matters most. Saturation tells you how much oxygen the water can actually hold at the current temperature—and that’s exactly what changes significantly over the course of the year.
| Water Temperature | 100% saturation in freshwater | 100% saturation in seawater (35 ppt) |
|---|---|---|
| 5 °C | 12.8 mg/l | 10.1 mg/l |
| 10 °C | 11.3 mg/l | 9.0 mg/L |
| 15 °C | 10.1 mg/l | 8.1 mg/L |
| 20 °C | 9.1 mg/L | 7.4 mg/L |
| 25 °C | 8.3 mg/L | 6.8 mg/L |
Values calculated at standard pressure according to Benson and Krause. They decrease with increasing altitude.
This leads to the real problem in summer: At 25 °C, even fully saturated freshwater can hold only about 8 mg/l, and at the same time, the fish breathe more rapidly. The margin between saturation and the critical value is then much smaller than at 10 °C. If you only look at the percentage, you’ll overlook this.
When to measure: timing is key
A single reading taken in the morning is not very informative. What matters is the lowest value of the day, and depending on the fish’s environment, this occurs at different times.
- Ponds with plants and algae: During the day, they produce oxygen; at night, they consume it. The oxygen level is usually at its lowest just before sunrise. So take measurements early in the morning—not just when you feed the fish.
- Tanks and recirculating systems: Without photosynthesis, this daily cycle is absent. Here, the oxygen level drops mainly after feeding, because the fish need significantly more oxygen during digestion.
- Flow-through systems: The outflow is critical, as that’s where oxygen is depleted the most.
In addition, you should take measurements outside of your regular schedule whenever something changes: during hot, windless weather; after thunderstorms; after an algal bloom; when new fish are introduced; after sorting or harvesting; when the inflow is reduced; and after any disruption to the aeration or pump. It’s also worth checking in the winter, since gas exchange barely occurs under a closed ice cover.
Where to measure: the most common mistakes
The most convenient measurement point is usually right at the aerator or the inflow, and that’s where the value is highest. For the fish population, however, the worst spot is what matters.
- Place it near the fish but at a distance from aerators, inlets, and oxygen injection points
- In larger ponds, feed in several spots, especially in corners with little current
- In deeper ponds, also measure near the bottom, as the water can stratify in summer and be significantly lower in oxygen at the bottom
- In recirculating systems, use fixed measurement points so that the values remain comparable over weeks
- Always record the value, temperature, time, and measurement location, because only a series of readings will show whether anything is changing
You shouldn’t simply assume that values well above 100% saturation are a good sign. They can occur, for example, due to heavy algae growth or the introduction of pure oxygen, and severe supersaturation can also harm the fish.
Which Sensor Is Right for Your Operation
There are essentially two principles for on-site oxygen measurement. You can find a detailed comparison of all methods in the guide “What Is Dissolved Oxygen?” For fish farming, the main consideration is the effort required in day-to-day operations.
| Polarographic | Optically | |
|---|---|---|
| Example | SX716 | DO850 |
| Preparation | About 5 minutes Polarization | No polarization |
| Maintenance | Replace the membrane cap and electrolyte | No membrane, no special solutions; sensor lasts up to 8,000 operating hours |
| During measurement | consumes some oxygen itself, therefore requires an inflow | Stable reading in 1 to 3 minutes |
| Suitable for | smaller operations and occasional checks | daily measurement rounds at multiple locations |
If you check several ponds or tanks every morning, the optical sensor will save you time on maintenance and waiting. The DO850’s cable is 3 meters long, which is long enough to reach into tanks and along the edge of a pond; for deep spots from the shore, you should check the cable length beforehand. If you need to document a series of measurements—for example, for clients or certification purposes—the DO8500 also offers a GLP storage function and data output.
If you want to measure more than just oxygen with a single device, there are combination devices available: the SX725 also measures pH, and the SX736 measures conductivity and salinity as well. All compatible devices can be found in the “Aquaculture Measurement Technology” category.
One thing should be clear: A handheld device only takes a reading when someone is using it. In densely populated facilities where a pump failure can quickly become dangerous, it is no substitute for a permanently installed monitoring system with an alarm. There, it serves as an independent check and for cross-checking the fixed probes.
To ensure accurate measurements
- Calibrate according to the instructions and before important measurement series
- Wait until the reading stabilizes, and do not record the first reading
- Set the salinity correctly in saltwater and brackish water: For the DO850, enter it manually; otherwise, the device will display mg/l values that are too high
- Keep the sensor cap clean, as algae and biofilm can skew the results
- Rinse the sensor with fresh water after using it in saltwater
If the value drops suddenly
If the value drops into the critical range, the first steps to take are those that have an immediate effect:
- Turn on the aeration or oxygen supply and check for malfunctions
- Stop feeding immediately; this reduces the fish’s needs
- Increase the fresh water inflow if enough good-quality water is available
- Postpone sorting, harvesting, and transport, as stress further increases consumption
- Remove dead fish and uneaten feed, then monitor the situation closely
Only then is it worth investigating the cause, and if the problem recurs, it often stems from the balance between stocking density, feed quantity, and aeration. If the fish continue to avoid the feed or die despite good parameters, the case should be referred to a fish health specialist.
Conclusion
Measuring oxygen levels correctly in fish farming is primarily a matter of routine: at the critical time, at the worst spot, and always in conjunction with the temperature. A reading taken at noon next to the aerator may be reassuring, but it doesn’t tell you much. Our article on Dissolved Oxygen in garden ponds shows how oxygen fluctuates on a smaller scale; the basics can be found on the “Dissolved Oxygen” page.
Frequently Asked Questions
How much oxygen do fish need in fish farming?
As a rule of thumb, at least 6 mg/l is required for cold-water fish such as trout, and at least 5 mg/l for warm-water fish such as carp. These are guidelines. Actual requirements increase with temperature, stocking density, and feeding.
When should I measure the oxygen levels in my fish pond?
It’s best to do so early in the morning, just before sunrise, because that’s when oxygen levels are usually at their lowest in ponds with plants and algae. In tanks and recirculating systems, the time after feeding is also important.
Why is the Dissolved Oxygen level lower after feeding?
Fish require significantly more oxygen for digestion. In addition, uneaten food and waste products consume additional oxygen as they break down. In densely stocked systems, the oxygen level can therefore drop noticeably after feeding.
Is 100% saturation always sufficient?
Not necessarily. At 25 °C in freshwater, 100% corresponds to only about 8.3 mg/l, whereas at 5 °C it corresponds to about 12.8 mg/l. Since fish also need more oxygen in warm water, the buffer is significantly smaller in the summer.
Optical or polarographic sensor: which is better for fish farming?
For daily monitoring, there are many advantages to using an optical sensor like the one in the DO850, since it eliminates the need for membrane replacement, electrolyte, and polarization. For occasional checks, a polarographic instrument like the SX716—which is less expensive to purchase—is often sufficient.
Can a handheld meter replace continuous monitoring?
Not in densely populated circulation and flow systems, because it does not issue a warning if a pump fails at night. In such systems, a permanently installed monitoring system with an alarm safeguards the system; the handheld device is used for checks and for measurements at varying locations.