Good grief.
Your pool store is creating a problem . . . and then selling you more stuff to fix the problem they've just created. To be fair, they probably don't know that they are creating the problem. Potassium monopersulfate tests as "combined chlorine". So, when your chlorine is low, and you add MPS (monopersulfate), you appear to have the very "combined chlorine" you were trying to get rid of! Glory be! Better add some more MPS; you've got the dreaded "chlorine lock" . . . whatever that is. (It varies from store to store, and region to region.) As CarlD likes to say, you've been "poolstored"!
Whatever. We need to move on -- but stay away from that store, except to get things you can't get elsewhere.
Are you sure you've got 40,000 gallons? That's a big pool, bigger than a standard 20x40 with a deep end.
Assuming that it IS 40K, a gallon of 6% chlorine bleach will add about 1.5 ppm of chlorine to your pool. Or, since your CYA is fairly low, you can use dichlor (sodium dichloroisocyanurate - 63% available chlorine) granules.
If you use bleach, add 4 gallons each evening, till you are holding chlorine levels of 5 ppm or better. If you use dichlor, add about 3 pounds for about the same results.
Do NOT add anything else, EXCEPT borax (20 mule team, white box, laundry section) if you need to raise your pH.
Reply with info about your filter, your test kit, and what happened in the week preceding the cloudiness. But meanwhile, get the chlorine up and keep it up!
PoolDoc
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