"While it is true that if the detector goes off there is no doubt that you have a stray voltage issue most of them are only sensitive down to 50 or so volts and some of the cheaper ones are even higher than that. It is possible, depending on the source, to have a stray voltage on the tank that doesn't set off the detector."
Voltage really isn't the issue - current is. If you have any submerged electric fixture in your system, then in proximity to the wires leading to that fixture, you'll have an measurable electric potential, which decreases rapidly with distance from the insulated conductor. So you have a voltage source, but due to the insulation (resistance), that voltage source can't source any current, and the field strength in the water will drop off very quickly with distance.
Put a nick or break in the insulation, remove the resistance which exists between the conductor to the water, and the water will conduct current through the water column and the whole body of water becomes a conductor which carries with it the full electric potential, 120 V. If you use AC voltage sources less than 120V, with sufficiently low internal resistance and these sources come into contact with the water - then yes - the AC voltage detector shown might not catch these.
I don't have any such AC voltage sources in my system and I'm not aware of any in typical use in reef systems. I'm not saying there aren't any - just I've never encountered one, and hence, have never seen a true, low impedance fault less than 100V.
I would not ever recommend a grounding probe without a Ground fault interrupter circuit.
A GFI circuit compares the current flowing on the hot side of the AC outlet with the current flow on the neutral side of the AC outlet. If there is even a small difference in current between the hot and neutral, the GFI will open the circuit. If you have a voltage in your tank the ground probe will send this current to the third prong on the AC outlet - not the neutral. Since this current is coming from the hot-side and is being returned on the third prong (safety ground) there will be a current difference between the hot and neutral and the GFI should trip. If you don't have a third prong, you can still use a GFI. Tie the third leg (safety ground) to line neutral. Any current shunted to any other ground (through your wet feet on the floor for instance) will cause a current differential and will trip the GFI.
If you don't have a GFI on the outlet, you won't be protected by a ground probe. It is very likely to cause more damage than good. Adding a ground probe to a non-CGI circuit guarantees that any voltage in the tank also produces a current and its that current which is damaging to the tank inhabitants. That current may be significant yet not be enough to trip your breaker. Ask yourself, have you ever seen a power strip burn before it trips the breaker? I have - quite frequently.
