[mike31154]

Three prong plugs are generally used on higher power equipment such as drills & tools of that nature, especially if they have a metal case. It's a safety measure to protect you from shock if the case of the appliance is somehow shorted to the power lead. A GFCI device will also protect you in a similar manner, even if the equipment only has two prongs. Your aquarium equipment should all be connected through a GFCI. It's code these days for any circuit in a damp area or dealing with water.

Salt water is actually a mild electrolyte, i.e. similar to battery acid. If you have two metal objects in close proximity in a salt water solution, there is a potential for electricity to be produced, voltage & current. Your pH probe is metal..... Don't stress out too much. I understand you need the pH probe to work with your controller, but at the end of the day, many folks are quite successful running their tanks with fewer bells & whistles.

Voltage itself poses no danger until it is given a path to create current flow, such as your hand in the water & bare foot in a puddle of water. The voltage would need to be quite high to complete that circuit through your body & zap you. In the tank it's unlikely any electrical current is flowing especially if you can put your hand in the water & don't feel a tingle or shock.