SEN0161-V2 pH Senors floating readings

Hello all. This is my first post and my first Arduino adventure, so please bear with me should I say something incredibly wrong or stupid.

I bought an Arduino Uno in order to build a simple fishtank monitor by reading a DS18B20 thermometer and a SEN0161-V2 pH probe and showing the values on an LCD. Attached you can find the schematics of the project.

The circuit should be now final, but I'm having a weird behaviour with the pH probe: while outside of the tank, and with very small amounts of water, the readings are consistent, when I put the probe in the tank for the final deployment I get different values every read (+/- 0.5 pH), basically bouncing between each others. As I said, when I was calibrating the probe and writing the code the values were a lot more consistent, with an average floating of about 0.03 pH, which I guess is a decent error for a low end probe.

What can I do to troubleshoot my issue? There's clearly something I am missing, but again being pretty much a newbie with Arduino and electronics in general, I really don't know what to do right now.

Thanks everyone!

I can guess that the water in your tank is being circulated by a pump of some kind.

Dip water out of the tank and into your calibration container and test that water. Is the reading stable? If so, use that value. Then you can average the values you find in the circulating tank water.