A fish tank pump can be either an air pump, for pushing air through an airstone or aeration tubing, or a filter pump, which draws water from the tank through a filter and back into the tank. Air pumps do not require priming. For water filtration pumps, you will need to prime the system to get the water moving. Very few aquarium filter pumps are self-priming. To prime means to place water in the portions of the filter that should be filled with water during normal operation before you turn on the motor.
How to Prime a Fish Tank Pump
Turn off the pump. Running the filter pump without water circulating through the system will burn out the motor.
Using a cup, pour water from the aquarium into the pump reservoir until full. For the hang-on-the-side type filter, the water may begin pouring back into the tank.
Turn on the pump motor. Continue pouring water into the filter until the water flows on its own. This may take several seconds.