Some water pumps put out quite a bit of heat. Because of this, when they are used inside the tank or sump, they can significantly increase the temperature of the water. This is especially true if the aquarium is of a smaller volume. Some pumps are worse about heating up the water than others, but to prevent this from happening you can run the pump externally. Assuming the pump is designed for external use, which many are, you can completely remove the heat source from the water. Of course there are potential problems associated with running pump externally, but you may decrease the amount of evaporation from the tank or reduce the water temperature with a simple modification of your plumbing scheme.