Turns out it wasn't the pump.
I was still getting voltage readings in the water.
I have some sort of grounding issue with my outlets - I have an electrician scheduled to come out, but for now I'm using two methods to ground.
One, a titanium grounding probe.
Two, I'm running another lead from the sump, to the carpet. Yes, to the carpet.
Apparently I've got a "loose" ground or something, because everything wants to ground to the floor. I get a reading of around 6V AC
by testing from the tank or sump, with the other lead jammed into the carpet. There's nothing when the ground lead goes to a "real" ground, on my power strip or outlet. That means that ground isn't grounded
With my second grounding circuit going from the sump or tank, to the carpet (wedged under the lip of the tank stand), I get zero readings from the tank to any ground.
Yes, this is a mess. Hopefully the electrician will be able to fix.
It's not any one piece of gear. I've unplugged everything, then added stuff back one piece at a time - voltage from tank to ground rises a tiny bit with each piece of gear.
To test my theory:
I take my skimmer pump, put it in a bucket. Test for voltage from the water to the outlet ground - nothing. Test from the water to a damp spot on carpet - I get from 6 to 30V AC
Move this setup to another part of the house - nothing - no voltage detected anywhere.
So basically, I threw my old 575GPH pump away prematurely (even though it was swollen up on one side).