Great info, thank you so much! So yes, I did not only turn everything else off, I also unplugged it - thinking the same thing you mentioned. The pH still jumps to 25 as soon as it's in the aquarium water, but tests fine when I put the probe in the calibration solution sitting on top of my aquarium lid in cups.
To answer your question, yes - I have a breakout board inbetween the Leviathan and the probe itself. For the time being I'm using a solderless breadboard for all of my sensors. This works perfectly on my counter and at the aquarium, it's only when the pH probe is IN the aquarium water that I get the crazy readings.
Just for giggles I ran a ground line from my solderless breadboard and put the other end in the aquarium water. Guess what I get? Perfectly accurate readings. What the heck does that mean?
I'd rather not have a ground wire shoved in my tank, but if I have to do so I guess I can use stainless or titanium or something. I have each sensor on that breadboard with it's own dedicated ground, even though back at the Pi/Leviathan it's a floating ground with continuity between all grounds (tested >4 ohms resistance which is the same reading I get when touching the multi meter probes together, essentially saying these are all connected with no resistance).
Any ideas?