Make sure that your filling the cannister up to the line on the side with water ...then start the pump for about a minute or so... turn it off and wait while the cannister fills. It's a good idea on established systems to tie a a fine mesh filter bag on the intake (or outlet depending on the cannister) hose to catch the bulk of the debris that will come out as the trapped air escapes. Remove the bag once the can's full and restart the motor.