Electrical exposure causing death is referred to as electrocution.
Improperly sealed pumps, aerators, or other electrical devices may result in electrocution, although this is rare if you are using today's ground fault circuit interrupters with 110 volt systems. 220 volt circuits can be more dangerous. Deliberate exposure of fish can occur as a malicious act, or during the use of electro-shocking devices which can serve to capture, subdue or kill fish. The only known natural source of high energy electricity is lightning, and while recorded incidents of strikes to fish are infrequent, the results can be devastating especially if valuable breeding stock are involved. The amount of electrical current necessary to cause death depends upon many factors, but especially the specific conductance of the water.