I have a 100lb sand filter and a 1 HP pump and I just add about 1/3 cup of DE each time, I WAIT 20 minutes( per instructions here) and check the pressure, if it has gone up 1PSI then I am done. I rarely have to add any more... the first time I did it, I did the same thing kept adding and didn't see any result in PRSure rise, came back and read this site and did it right after that!!
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