If you spend some time with google your head will explode on the pros and cons of using a ground probe.
After a ton of reading (and an electrical engineering background) I'll offer my two cents.
Cent #1: Install a GFCI breaker or outlet on all devices used around an aquarium. While there are oddball situations where you can still be shocked, it is the best single human safety device you have going. Same for any outlets you use for extension cords for water transfer pumps, etc. Water + People + Electricity is always better with a GFCI breaker.
Before cent #2 consider this: Birds sitting on a 14,000 volt bare wire live. Go outside and look, the top wire on many residential lines (in the US anyway) is in that vacinity of voltage, and almost always bare.
The reason is that it is current, not voltage that harms. The shock on a door knob you may feel is usually several thousand volts, just almost no current. Voltage is a charge, current is flow of charge. With no where to go (generally called "static electricity") there is no current and no harm, so the bird lives.
Cent #2: I decided not to install a ground probe as MOST current in an aquarium is going to build up from 'static' sources, such as water movement through plastic, and not through leakage from line voltage of equipment.
MOST line voltage equipment is well designed and does not leak current. If yours does, throw it away, do not "fix" it by giving it somewhere to leak to.
If your tank has this (e.g. you walk up and touch it, especially with a cut on your hand, and feel it), but does not trip a GFCI breaker, it is probably just such a build up. Left alone, with no ground probe it has no where to go, and just sits there -- like the bird on a wire. Provide a ground probe and IF you have this type of otherwise static charge being continually generated, you create a current flow in the water you otherwise would not have. You no longer feel the shock when you touch it -- but fish in the water are continually having a small current flow through them from the source (e.g. filters) to the probe.
If you want to think of this in aquatic terms instead of electrical, try this. Think of a very, very tall container, and a small water pump, like a power head. Now imagine it filling up that container by pumping water in from the base. It will fill until it gets as tall as the max "head" pressure of the pump, then stop filling, even though the pump keeps running.
That's surprisingly similar to friction generated (and similar) electrical charges in a tank -- they build up until they reach a certain potential, and can build no more, even though the source is still chugging away, trying.
Now back to the tank -- poke a hole in the tank near the bottom for the water to run out. The small pump can then continue pumping, and there is a flow of water from the pump through the tank and out the hole.
This flow of water is, in the analogy, electric current flowing into the ground probe. Without the probe (no hole), the build up of charge stops, and no current flows, just as no water flows in the water case. With the probe (hole), there is some continual flow of water, i.e. current.
Now I offered it as 2 cents, but you paid nothing for the advice, and that is exactly the guarantee you get with it. Read the volumes of pro and con and make your own mind up.
