Hello Pit, thank you for feedback

After reading your message I performed the following test: I filled a 1-liter pot with water from the aquarium and I left it in a table near the aquarium. Then I placed the pH probe in the pot. Same water, no other electrical device in it. I left it for 15 minutes, the measured pH value was the same.

In the same pot (and in the aquarium) Hanna meter is reading 0.4 more (6.5 Hanna / 6.1 GHL). The drop checker colour is consistent with the Hanna checker (green with a bit of yellow). CO2 concentration at KH 3.5 is 33 ppm at pH 6.5 and 83 ppm at pH 6.1. The drop checker colour and observed fish behaviour (completely normal) are consistent with 33 ppm CO2. When I have performed direct CO2 concentration tests (chemical) at the same hour, the values have been around 40 ppm.

Before returning the pH probe to the aquarium I did another test. I cleaned the probe with Hanna Cleaning solution and then I placed the probe again in the pot. After 5 minutes it was reading again 6.1.

After searching documentation about pH drifting, I have seen that possible causes are: electrical potential in the water, clogging at the ceramic junction, and also low conductivity sampled media. My water is 185 ppm TDS (370 uS) it is soft freshwater very different from seawater, I wonder if it can be the cause. In the documentation also says that with low conductivity media you get wrong values but you get perfect readings if you place the probe in calibration fluids, this happens to me as well.

I wonder if I should be using a different pH probe. I will post the feedback requested by GHL at next calibration (maybe next weekend) and let’s see.