I really dont think that is from the air vent. I wouldnt have made it any bigger, either, but what is done is done. Put some tape or your finger over, and see if that pinpoints it to the air vent. Is it possible the pump is sucking air somewhere else? Fluid dynamics indicate that bubbles would be pre pump, not post- which is why I am asking. If it does turn out to be definitely coming from the air vent, just plug it up with some silicone or superglue, You'll just need to make sure the end of the locline is right at or JUST below the water line, otherwise you'll siphon way too much water back to the tank.