I don't know much about the technical aspect of GFIC's but I do know that when I noticed my power head in my tank wasn't working I saw that the GFIC had been tripped. So I unplugged the cord, reset the GFIC and then plugged the cord back in again, and again it tripped the GFIC, the breaker the GFIC was on never tripped, it only tripped the GFIC.
When I finally looked back behind the tank and examined the power cord, much to my shock this is what I found:
All I have to say is thank God that the GFIC tripped, because the breaker it was on never did. Also very thankful that the only thing that got fried was the power cord and power head. I'm guessing water splashed out of the tank and onto the extension cord, causing a small electrical fire (which you can see the damage done to the cords in the above pic.'s) causing the GFIC to trip. Needless to say IMO if the GFIC hadn't tripped (or if I didn't have one) I'm guessing the damage would of been a lot worse!