i had a similar experience just a couple of days ago with some laboratory sea urchins that got nuked after they chewed through a filter wire. the GFCI did not trip and the current ran through the tank for who knows how long. in addition to the GFCI, it is also important to have a ground--that is the reason that the device did not trip for me. i am going to get some of those titanium ground probes for each tank.
This is correct.
GFI's will not trip under a high amp draw unless current has left the circuit (through ground). That is what circuit breakers are for, and even then you have to exceed 15 or 20 amps (depending on the breaker) and that is a lot...most vacuum cleaners use 10-12amps.
I have seen a few posts regarding GFI's not tripping and as far as I can tell, in each case while a device was broken, current entered the tank through the hot wire and left through the common line, thus the GFI did not see a leak. There needs to be a ground probe in the tank along with the GFI for an ungrounded device to trip the GFI.
Note if you have a grounded device (most pumps), they will trp the GFI. But ungrounded devices (cheaper powerheads and most heaters) will not as the only way out is through the common.
I strongly recommend the GFI and grounding probe.
Also, I can comment that I have had one glass heater break in my 20 years and it was in a freshwater tank (thank god) and not saltwater. It was not Jager and since that is the only brand I use. I have had Rio's go out and not trip the GFI until I stuck my hand in the tank...the Rio killed everything.
So for my next tank, GFI's, Grounding Probe, Jager Heaters, and no Rio's.
As far as installing the GFI, go to your favorite hardware store and purchase a surface mount box, a GFI outlet (or two) and an appliance chord (ground of coure). Mount to the wall or stand. Follow instructions on wiring and plug into your favorite outlet (or the one closest to the tank). Since your will not be hardwired into the house electric, you are fine with codes. Cost should be less than $20 for one GFI.
Chris