GFI stands for ground fault interrupter. That means that the plug has a "mini-breaker" in it. If there is any current that flows to ground, it will trip. Even a little current will make it trip because water and electricity dont mix. Electricity flows from hot-neutral/positive-negative and so on. On a plug, there are usually 3 prongs kind of in a triangle with the bottom prong usually being round. The two flat prongs are the hot-neutral/positive-negative. The round prong is the ground. In homes, by the Electrical Code, everything has to be grounded. Bathrooms and outside plugs must be GFI because of the risk of moisture/water that might create a shock.
It is most advisable to use a GFI plug when the application to close to moisture/water. Something else important is that everything be grounded. Most pumps/heaters/filters etc. have a ground (the round prong) so it is nothing to worry about. However, the plugs/receptacles must be grounded in order for it to work properly.
There is no such thing as 'stray voltage' in a 120V system. The reason is first of all, everything is grounded. Second of all, the voltage would be so small it would not be noticeable. And third, if you do have a 'stray voltage', you have a problem with equipment or wiring and it must be fixed anyways.
In nature, in rivers, lakes, and oceans there are naturally occuring currents (electrical) because the earth has a magnetic field. My post is getting far to long so I will not get into it.
If you have a salt water tank, it will not hurt to use a ground probe. Accidents can happen and saltwater fish a much more expensive. Freshwater it would be useless as freshwater has a lower conductivity as saltwater. Pure water has no conductivity because of no impurities.