What Is Going On Here?

[blindcavefish]

My pump, in the corner filter of my Juwel 70 litre keeps blowing out air?

Every 3 seconds or so, I hear a strange gravelly blowing noise and then it pumps out a load of little bubbles - any ideas?

 

[hensonc4098]

If you cleaned it recently, it could just be a little air-pocket inside it that is slowly working it's way out