It was actually a post of yours that got me to use GFCI. I setup 4 separate gfci lines that all controlled by my apex so I should get a notification of loss of power to those outlets should it trip. Thanks again for the build info on this DIY project @Brew12Thanks for the invite!
I actually think this is a bad example to use for fish in our tank. In a salt water tank the water is more conductive than the fish. It would be as if the fish were living in the ground wire. Same reason I hate the bird on a wire analogy for reefing. It just doesn't work.
This is the main reason I recommend running a GFCI with ground probe. Odds are that the exposed wire was copper. Copper will corrode more quickly with electricity running through it and release that copper into your tank. It will also generate heat which can melt the plastic and release other contaminants. I want that failed pump/heater/device de-energized as soon as it faults. It would be hard to electrocute a fish, but you could easily kill them with contaminants.
That one is tougher. If you were wearing well insulated shoes and didn't touch anything else but the water odds are it wouldn't trip. If you were barefoot on concrete it should trip. Hard to say for anything in between.
Umm... I would say no. A GFCI is a great protective device but anything can fail. I see this situation like trying to cut your hand through cut resistant gloves. It's not what I would call safe. Hopefully the glove would stop the blade of the GFCI would trip, but I wouldn't test either on myself on purpose.
Many people who are against ground probes run them without knowing it. Most titanium heaters have a ground wire connected to the titanium housing. Some titanium heaters only have 2 wires and I've found some 3 wire versions not wired correctly, but by and large they are.
I will say that I won't run my system off of a single GFCI. My return pump and power heads are always off separate GFCI's. My display would be fine for 12hr+ with either of them tripped. If I lost everything it would deteriorate much more quickly.