The water clarity of the output from filter 4 has been getting worse and worse up until 2 days ago. An air vent on the top of the barrel was added after I noticed the output pulsing while the pump was in operation. I realized at that time that the pump was actually drawing water from the filter’s output because the water had reached the top of the barrel beyond the overflow drain just below the top and was forming a seal so as to allow the pump drawing water to create a vacuum strong enough to briefly cause water to be forcefully drawn through the filter much faster than it could handle. This was (apparently) causing the increased turbidity and (possibly) the biological contamination of the output water. So. . . know that:
The storage container, especially when used with a pump, must be vented above the water level in the container or the pumping action from the small recirculation motor (or any pumping action) will actually forcefully draw water from the filter at a destructive rate.