Pond Pump impellers will need periodic maintenance and cleaning and it is especially important to check the impeller if you have a problem with your pump.
This video from Hozelock shows you how to clean an impeller in a Cascade Pond Pump.
Although the video specifically demonstrates how this is done with a Hozelock Cascade Pump the principal is very much the same for most makes of pond pump.
Pond Pump Impellers are a wearable part with many variables that can affect their life. It is essential to keep pond strainer covers clean as blockages caused by a build up of blanket weed or debris can cause the pump to overheat leading to an increased build up of lime scale and possible fracturing of the pump impeller.
Visual & Physical Impeller Check
Visually check the impeller barrel to make sure it has no cracks or damage. Then hold the impeller barrel between your fingers and rotate the impeller fins. The impeller fins should rotate freely for about a half a turn and then lock. If the fins can be rotated continuously through 360 degrees then the impeller is probably worn and you will need to replace it. This kind of wear is usually caused when a pump is turned on and off frequently. Please note pond pumps are designed to be run continuously.