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pH inconsistent readings

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Hi all, 

Seems all I'm doing is posting problems on here at the moment. Ho hum, hopefully things will stabilise soon so I can get back to just enjoying my new tank. 

Anyhoo, had a bizarre situation come up over the weekend whereby my Dupla electrode starting showing a huge drop in pH but my pH pen and permanent colour tester in the tank are showing closer to expected values. 

Some background: 
- Tank [1220x930x800] filled with water and cycling (with daily top ups of Stresszyme) commenced mid-December (additional sump is 1800x400x400) 
- Tank has a lot of plants 
- Cherry Shrimp introduced a couple weeks later and doing quite well 
- High tap water pH reduced by automated CO2 injection plus IAL's in sump and tank 
- pH usually between 6.8-7.1 
- Water changes done via RO water into a water tank; shrimp gH/kH+ added to remineralise during water changes (along with Stresszyme) 
- No additional livestock put in until Australia Day last week. Livestock added: 7 discuss; 12 rainbows; 30 cardinals (had a few cardinal deaths since then and one rainbow was a little dodgy but is in quarantine now and doing much better; discus are doing great!) 
- On Saturday I added 4 blue rams and a big bolbitus 
- Other water parameters (that I measure): TDS: approx. 300-350; Ammonia: nil 

- <Update>: Got home and tested gH and kH: Results = gH 11 and kH 7
- Electrode is in the sump in the final partition where the main pump, chiller pump (inflow and outflow) and UV pump (inflow and outflow) all are as well. 
- Distance from main pump (sump) to tank is about 10 meters. Sump is set up in an external (plastic not metal) shed and doesn't get much direct sunlight. 
- CO2 goes directly into tank (not the sump) 


Yesterday I started getting very strange pH readings via the Dupla electrode. Since Thursday it's consistently been 6.9-7.05 but then dropped yesterday to 6.4. I recalibrated it and tested it a few times with calibration solution and it seems fine but the actual water readings seem out. I did a 25% water change and I know my water tank water has higher pH so the readings should've gone up. pH slowly went back up to 6.7 during the water change but then dropped to 6.1-6.2. 

I then got my pH pen, recalibrated it as well and it showed 7.5; it's usually 0.3-0.4 above the Dupla electrode. 

The permanent pH colour tester inside the tank was showing 7.2-7.4, much closer to the pen than the Dupla electrode. 

I thought maybe the heat of the day was affecting the electrode but the water temperature was consistently around 28-28.5. 

It's just very strange that I'm getting such different readings across the 3 devices, with the electrode showing a fast and significant fall. Given the pen and permanent colour tester are close to each other and show what I'd be more inclined to expect, they seem more correct. That suggests the electrode maybe out. But when I put the electrode into the 7 and 4 pH calibration fluids, it hits them right on. 

It's been suggested to me that all the pumps in that section of the sump maybe circulating the water too much for the electrode or that the magnetism of the main pump maybe influencing the electrode but if that's the case, it would've been happening before not just all of a sudden. 

I thought maybe the cooler water from the chiller may have been throwing off the readings but checking the reading all through the night until about 12am they remained around 6.1-6.2; although it was a hot night and the chiller kicked on and off throughout the night. 

<Update>: Electrode is definitely wonky. Got home today and it's reading pH 4.9-5.2, with heavy fluctuations each second of anywhere up to +/- 0.06. Moved it out of the sump and into a quarantine and same thing still and quarantine tank doesn't have circulating pumps or a chiller.

I'm now looking to order some pH reagents from Hanna Instruments so I can fire up my photometer to test the pH with that to get a very accurate reading. I've also closed off the CO2 until this is sorted. 

Has anyone had a situation like this? Any thoughts?

Edited by kapp
Test results for gH and kH and new electrode readings

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Hope you sort it out.

Can you just replace the electrode with a new unit?



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I have heard of similar problems before with these type (not that particular brand) that have issues when it comes to recalibration. The exact reason I am unsure but I tend to agree with JayC.

Or reading everything you are going through to apply an age old theory which is the KISS principle. At the end of the day sounds very painful especially if only keeping cherries. Also if I read correctly it is still only a couple of weeks old and IMO will definitely still be cycling which combined with Co2 (Not really a fan) both of these can cause fluctuations in ph levels and these can also change throughout the day. I liked to do my testing at similar times when eveer checking so that there was consistency. Like I said just my opinion.

What I will also say is the most important thing I learned over many years the hardway.... KISS 

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