Here's a basic way to create a SW battery:
http://www.miniscience.com/projects/airbattery/index.html">http://www.miniscience.com/projects/airbattery/index.html</a>
I can only explain by my experience. I had a low voltage lamp fall into my sump for probably 4-6 hours before I found it and removed it. The breaker nor the power strip never tripped. Over the next week or so, I lost every single invert in my tank. None of the fish were affected by it by the inverts sure were. Now keep in mind this was pure electricity being fed into the by something that wasn't designed to be water proof.
And no I didn't have a GFI outlet nor a grounding probe at the time (though I have both now).