What you describe is very, very strong indication of missing sand.
This is commonly a result of matching an undersized sand filter with an oversized pump, which results in blowing sand out with every backwash. Unfortunately, this pairing is common, because oversize pumps are 'sexy' (BIG horsepower!!) and don't cost much more than smaller pumps, whereas nobody gets excited at sales-time about a 30" filter instead of a 24" filter.
Verification test? Go to Walmart, etc and buy some DE filter powder. Add a few cups to the skimmer when the pump is on. If a white stream emerges from the returns . . . you've verified non-filtration, which is almost always lack of sand.
Short term solution? Add more sand.
Long term solution? Either get a bigger filter, a smaller pump OR replace the impeller on the existing pump with a lower HP rated impeller. (You can put a small impeller on the same model pump, so long as the HP of the MOTOR is higher than the HP rating of the impeller. BUT, if you put a big impeller on a small HP motor . . . you'll just burn out the motor. Job requires some skill, as well as a new impeller and seal. It's prudent to ALSO plan on replacing all case gaskets & O-rings.)
Alternative kludgey solution? Install a valve on the EFFLUENT side of the pump, BETWEEN the pump and filter, and throttle maximum flow to less than 15 GPM per sft of sand surface.
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