I know this thread is old, and honestly I only made an account for this reply as it's on the top Google results for aquarium grounding. All the complex replies in support of GFCI + grounding probe give good information, but as an electronic hobbyist that has to explain concepts to people all the time, it's not that hard.
If your tank does not have a path to ground, a GFCI won't trip if there is an electrical fault, UNTIL a nice path to ground, your arm, comes in contact with the water ONLY THEN there will be a fault current to detect for the GFCI. The fault current does not occur until there is a path to ground. With a ground probe there is a preemptive path to ground to detect the fault. GFCI measures current from line the neutral and trips if there is a difference, fault current can only go to neutral or ground, if it goes to neutral the GFCI does not detect an imbalance because there is not one, that is how current should be flowing, The GFCI only trips if it doesn't know where the current is going (to ground somewhere)
GFCI and no ground = only trips WHEN You get shocked, may save your life, but you still get shocked.
GFCI + ground in aquarium = GFCI trips if a fault occurs, as soon as it does, BEFORE you get shocked, given the fault current is sufficient.
Some of the replies from others that don't understand there can be voltage differential without current flow meaning a GFCI won't trip until there is a ground path to let current flow , hurts my brain.
Anyway, have a nice day lol
I was just looking to see if anyone actively grounds their aquarium and uses a GFCI as i just got a nasty shock from a busted heater in one of my freshwater tanks, and would not like to repeat that as half the heaters on the market are unreliable even if they are expensive name brands.