Sump levels constantly changeing?

[Lampshade]

Another Oddball suggestion, but depending on how the restaurant's wired there could be a voltage drop when the lights and everything else are on in the tank, and also the restaurant. There's obviously building codes that would provide sufficient wiring with no more than 5% drop, but... if the tanks pigtailed off another circuit that's feeding something else, there actually could be 10%+ drop, reducing the output of the return pump.

I had this happen with 2 portable AC's plugged in to the same circuit as my tank, also finding out that 15A breakers can take a little more than what they say haha. But my water level in my tank was changing by 1/2 an inch when the AC was on, and off again the water would come back up. Turns out i had 102V at the receptical, and 112 with just the tank. This is >5%, and also in a new house, which should be to electrical code.

I agree with the others, probably a plumbing issue, but an easy test if you have a multimeter.