Well i had an experiance not so long ago that may or may not shed a little light on this. First off i am an electrician so i know a little bit about what i'm talking about, I recently had a heater break in my sump and never trip the GFCI it was connected to. Originally i had thought that the GFI was bad but after some investigation and some thought this is what i came up with....Mostly all tanks and sumps are for the most part isolated from the ground and all the electrically run equipment in these systems are insulated to protect against voltage leaking into the water. Now that being said GFCI brakers or individual recepticals trip on overcurrent, short circuit, and an unbalanced potential between neutral and ground. If none of this stray voltage has a place to go "ground" then thats where stray voltage comes from and also protected or not the GFCI will not see the potential and therefore will not trip.