If you want to filter out fine stuff, it will probably take more than a cup, and you'll have to backwash quite frequently. But it is do-able

Use 1 - 2 cups to TEST your filter: if the DE leaks through and back into the pool (white cloud from return jet) your filter has problems and likely needs repairs.

AG pool sand filters are USUALLY undersized, so the pumps overpressure them and blow the dirt through. Test with DE first, and if the filter is OK, throttle back the flow by partially closing the discharge line somewhere, reducing flow rates. (You may have to INSTALL a valve to do this - but NOT on the suction side.) With reduced flow rates the filter is likely to actually remove dirt in the water as it flows through the filter.

If the filter is not OK, fix it or get it fixed. You can't operate a pool with chemicals alone.

Ben