I use a probe on my tanks. Just a couple of things to point out. First, a physics principle: The ocean is grounded. The water touches the ground, doesn't it? In any event, it's large enough to disperse considerable voltage on its own anyway. Our tanks, on the other hand, do not have that luxury(of being huge!) Glass, rubber tubing, etc. are pretty good insulators. They will maintain whatever stray voltage you have pretty well.
The problem with that voltage is predominately how it affects our tank inhabitants. If we do something unwise, a GFCI will keep us from frying. The animals have a different problem. This is the second issue, biophysics. Voltage differentials are critical for every biological organism. Remember action potentials from way back in Biology 101? Just one example that depends on voltage differences in and out of the cell. Same for our tank inhabitants. If we have stray voltage in the environment, they have to overcome that voltage to maintain their metabolism. It may not cause a big problem right away, but can have a long term effect, just as if your salinity were 1.050 or something. Exactly how it affects them depends on the organism and the amount of voltage. Why subject them to that stress just to save $10.00?
My $0.02. HTH, ed