Unexpected behavior of redox reading

Hello,

I am seeking advice about an apparently unexpected (or at least from my side not understood) behavior of ORP measurement. I have a 800 liter planted tank with discuss. Running with two Eheim 2080, CO2 injection via external Aquamedic reactors which are setup to support also ozone injection. In addition I have an Eheim UV lamp in the setup. Set setup is controlled by a PL4 with pH, redox, temperature and conductivity and a level sensor for ATO. The four sensors run in a sensor cell, not inside the tank. I am running the tank at 28C temperature, around 240 uS conductivity and around 6.4 pH. I did setup the tank in May.

The thing which does not work so well is the ORP measurement. I am attaching graphs from a 6 month period, 1 month period and the last week. After some initial time, the ORP measurement did go up to around 600. It does not go over the roof to 1000, which I guess would indicate the probe or interface has an issue, it stays around 600. Never the less 600 does not at all sound really possible to me. Note that the UV lamp and Ozone have been running only sometimes (e.g. they have been off for a week, then in service again due to some medication and anti-algae treatment over the time). Also note that the sensor has been repeatedly calibrated and I am pretty certain its OK form this perspective. The second part of the equation is the behavior when I add hydrogen peroxide and when I clean the sensor. In the first case I had to add 25ml 3% H2O2 to the tank due to algae issues (which helped really well against cyano). What happens when the H2O2 is added is that the redox actually slumps to below 300. Now let's disregard the actual chemical reason of the drop. What happens then is that within 8-10 hours the value will climb up with a repeatable curve to 600 again. This is puzzling, but would reinforce that the actual value is 600, however unlikely. Yet a bit different behavior was experienced over the last week when I decided to study the behavior more. I first cleaned the sensor (yes I understand there is so bur-in in period afterwards). You can see the behavior in the graph. After the first sensor cleaning on 20.Dec the ORP value did go down to around 330 and settled there for a moment. I turned on the UV lamp then - you can see that the ORP started to climb very slowly on 21.Dec. Note that before 20.Dec UV neither ozone have been on for around 2 weeks. On 22. Dec I activated also Ozone, which is running during the night for a couple of hours, you can see that the curve started to climb up more steeply to to 410 at 10am when I added H2O2 to deal with an issue. You can see the slump again to under 300 and going back up again (no idea why the double dip is there though). Now the absolutely puzzling thing for me. Afterwards the ORP did climb not to 410 or a bit more as a result of the H2O2 but straight up to 570. I added another dose of H2O2 on 23.Dec morning. The slump below 300 again and climbing up a bit faster now back to 570. Now at that time I decided to check what's going on. I took out the sensor, cleaned it again (on purpose) and calibrated it. Afterwards the reading back in the tank water was 320, going up afterwards to 330 and pretty much staying there for 24 hours (its still at 333 at the moment). To round up the picture I purchased a separate handheld ORP meter a week ago to work with my breeding tank and to test what's going on. This one is showing also 330 right now, very close to the PL4 ORP reading. Now - I am pretty sure if I add the H2O2 again the ORP will jump up to nearly 600 and stay there. I was also measuring with the handheld ORP meter after the previous doses of H2O2 with P4L reading 570, but did come to 300-ish thus having a significant discrepancy. I thus believe the real value is somewhere around 330-340. Please let me know if you have any idea what could cause this behavior of the ORP reading and from my perspective apparently wrong long term value after adding H2O2.

Thanks,

J.

Hi there,

The GHL ORP probe is going to be more sensitive to changes made in the aquarium and will therefore show the outcome sooner than the handheld instrument. If you have already calibrated multiple times and let the probe sit in the 220mV fluid and saw a result of 220mV, this verifies that the probe is working correctly. The behavior of ORP is a tricky concept to understand as there are many variables which can alter the end result; UV bulb age, supplements added, dosing schedules, etc. From your chart it seems 500 - 600mV is the norm for your tank.

ORP is commonly used as a general indicator for when a significant change occurs in the aquarium. There is no "recommended" range as each tank will have its own "normal" range. In your case, it is 500 - 600mV.
How does the tank inhabitants look to you? If they are all healthy, then this level does not bother them.