The GFCI will switch off if it detects "leakage" of current. Current leaks when a circuit is completed, either by you, when you have one hand in the aquarium and one or more feet on the ground, or by the grounding probe, if it has ANY path to the ground. Technically, the GFCI will protect you even if you don't have a grounding probe, because as soon as the current starts to go up your arm, through your heart, and down your leg, it will trip. But I prefer to let a piece of wire do this, just in case the GFCI is having a slow day. Yes, it will trip as soon as there is a ground fault. But you WANT it to trip as soon as there is a ground fault. Your fish will starve to death if you are laying on the floor in a puddle of salt water for three weeks, even if the pumps are still working. If you are worried that it might trip spontaneously, plug some of the pumps or whatever into a second circuit.