As previously stated the circuit breaker will not protect you from electrocution or death. Voltage does not kill (the 20,000 volt engine spark plug wire will not kill you), current kills, and typically 0.1 amp (100 mA) to 0.2 amp (200 mA) through the right part of the body is lethal.
A typical 15 amp home circuit breaker could require more then 10 times the rated current (150 amps) to trip quickly. So a 15 amp breaker at 10x current can potentially produce enough current too simultaneously to kill 1,500 people before tripping! The circuit breaker will not protect you. It can cook you like a smoking hot dog before it trips.
The GFI cleverly measures the current going out one of the flat openings of the receptacle, and subtracts it from the current returning into the other flat opening, By the UL standard, if more then 0.004 amps (4 mA) to 0.006 amps (6 mA) of the return current is missing (because it potentially got diverted through a person) the GFI will instantly trip. This amount of current is well below the lethal threshold. Needless to say, ALL of our equipment in the fish room should be GFI protected so we don’t inadvertently get electrocuted!
Assuming everything is on GFI protection, the ground probe question is a little more complicated.
On one hand, if there is a defective piece of equipment in the tank leaking current, the ground probe should provide a path to divert enough current to immediately trip the GFI.
But every piece of equipment will have some measurable leakage to it, especially as it ages and after soaking in seawater. It’s possible the ground probe induces a small current path through the tank that may be too small to trip the GFI, but large enough to have a negative effect on the livestock. It’s also possible the ground probe can provide just enough of a leakage path that can inadvertently or randomly trip the GFI when no real fault exists.
The answer to the ground probe question then comes down to personal preference.
Keep in mind the normal leakage current of each piece of equipment adds up, and there is a limit to how much equipment one GFI can handle until it starts randomly tripping when no real fault exists. This is where larger systems typically require multiple GFI devices to be reliable.
You definitely want to use high quality UL approved GFI devices, and stay away from low cost junk. Also testing your outlets with a plug-in GFCI Receptacle Tester is a great idea. This will verify the Hot, Neutral, and Ground terminals of the outlet are wired correctly. Also with a push of the button, you can verify the GFI trips and is working properly.
