Article written by: Jason Danaher Ph.D.
Keep in mind fish often show dissolved oxygen stress before oxygen monitors do. You should always observe your fish for unusual behavior like gasping at the water’s surface, signs of lethargy, or congregating around inflows to the tank or source of supplemental oxygen. If fish behavior and oxygen readings don’t align, then double-check your oxygen probe (Figure 1). Even good equipment gives bad data when misused or not properly calibrated and when it comes to oxygen concentration in your aquatic system there is no room for error by overlooking the details. Let’s briefly discuss some of the common issues operators encounter, why it matters, and some best practices or remedies that can be used to prevent repeat occurrences.
First, poor operational practices often include infrequent calibration and incorrect calibration methods. Small errors can shift readings by 0.5 – 1.0 mg/L. Management practices should include daily calibration for critical systems and at minimum weekly calibration of devices used around the facility. It is important to enter the correct water temperature and salinity depending on your meter and calibration process. Dissolved oxygen solubility is largely dependent on water temperature and salinity. Warmwater and saltwater hold less oxygen. Make certain your meter is equilibrating correctly for these two parameters. Also, ensure your “zero” calibration solution is fresh and not expired.
Figure 1. A battery operated handheld oxygen meter with a polarographic probe, additional membranes, electrolyte solution, and calibration equipment.
Second, for electrochemical probes like polarographic and galvanic, do not ignore the condition of the membrane. Torn or wrinkled membranes should be replaced. Electrolyte solution should be replaced if air bubbles are observed under the membrane. Refer to your oxygen meter’s manual on how often to replace the membrane and electrolyte solution regardless of condition. Best practices are to inspect membranes routinely, replace membranes when they are worn, and fill electrolyte solution carefully to make certain an air bubble cannot form between the solution and membrane.
Next, when using polarographic and galvanic probes ensure there is sufficient water flow around the probe while measuring oxygen. Electrochemical probes consume oxygen during the measurement process as the oxygen molecules present are reduced at the probe’s cathode. Measuring oxygen in still water or not moving the probe in the water column can result in a false, low oxygen concentration reading. Conversely, the wrong monitoring location can result in a false, high oxygen concentration. Avoid monitoring oxygen around aerators, supplemental oxygenation systems, or tank inflows. Best practices are to measure oxygen in areas where there is water movement and gently move the probe during spot checks. Avoid tank corners and centers. Be aware pond oxygen concentrations can often experience diurnal fluctuations between early morning and late afternoon readings.
In addition, improper storage can lead to probe fouling. Biofilm buildup, algae, or solids on the probe will prohibit an accurate reading. Maintaining a routine maintenance schedule and gentle cleaning of the probe will reduce the occurrence of this scenario. Also, allowing a polarographic or galvanic probe’s electrolyte solution to dry out and poor handling of optical caps associated with optical probes are common neglects. Following the manufacturer’s guidelines to protect sensors and using storage caps appropriately are part of best management practices.
Finally, relying on a single probe may be bad practice for aquatic system operators. Probe drift may go unnoticed. Probe drift in oxygen measurement refers to the gradual change in a sensor's output signal over time, even when the actual oxygen concentration remains stable. Cross-checking and spot checking with a secondary probe that has been properly calibrated and maintained is a practical method to confirm accurate oxygen concentration readings. Probes age and eventually even probes that have been properly calibrated and maintained will need to be replaced.
Your dissolved oxygen probe is not just a tool - it is a life-support monitor for your fish. It can provide accurate oxygen readings with daily calibration checks, weekly cleaning, and recommended monthly service according to the manufacturer’s recommendations. Always have an annual line-item budget for recommended service and eventual probe replacement because you don’t want to leave your fish holding their breath. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com for additional information on measuring dissolved oxygen in your aquatic system and maintaining monitoring technologies appropriately.