I have found that calibrating with Hydros fluid (or any other brand for that matter) is when the problems begin. Surprisingly, I do have some stray voltage going on but my Hydros salinity probe is completely unnaffected, while my orp and both ph probes are.
The only way I can get Hydros salinity probe to not swing wildly:
Step 1 - toss the calibration fluid in the trash
Step 2 - clear calibration
Step 3 - buy a Tropic Marin glass hydrometer and a 1000ml graduated cylinder
Step 4 - read the artice and mix Randy's standard salinity solution found here
https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/
Step 5 - after your TM glass hydrometer is calibrated, write down the value and use the probe offset to set the probe to the TM hydrometer's reading
Step 6 - if it ever starts to drift, get out the graduated cylinder and again use the offset to set the probe to whatever the TM hydrometer is reading.
My opinion is these salinity probes should not be used as a definitaive, be all, end all salinty reading. They should only be used as an alarm bell of sorts, for instance, if your ATO gets stuck on, so it can warn you soemthing is wrong. Use the TM glass hydrometer as the definitive reading that you can and should depend on. DON'T depend fully on the salinity probe. It's just a warning something could be wrong.