Also get a grounding probe and put that in your tank. Basically it plugs into the wall using only the ground portion so it pulls any stray voltage in comes into contact too.
This HAS to be done or every one of your inverts is at risk. I had a low voltage light drop into my sump for a couple of hours. After pulling the thing out (with the power shut off to it), I never noticed any shocks or anything. Because it was low voltage, it never tripped my breaker. But by a week later (might have been less), every single one of my inverts had died. It was a brutal thing to see. It was then I checked the voltage with my meter and sure enough, a couple of volts worth was still in the tank.
If you feel like driving out to my house here in Acworth, I can lend you my probe if like.
