Because it was easier to copy and paste from the internet here is the following
Voltage without current; Voltage is the name for electric potential difference. Whenever there is an electric field, there's an electric potential difference (since the electric field is the gradient of the potential).
A simple point charge generates an electric field (and therefore a voltage), but in the absence of other charged particles, there will be no current.
Pumps can create the electrical field without any current.
I took some readings this morning from my office tank with a rio 1700 return pump, and a mj1200 skimmer pump. Total voltage 3.34 volts 0 current. If I left it on for a while .01 milli amps would show up which is .0001 amps. Also all new GFCI receptacles are now self testing, but it is still a good idea to perform a manual test. If you purchase a GFCI make sure it is the self testing, some of the big boxes may have old stock?