Read the Best Guess page, linked in my signature. That will explain the problem. ASAP, add a 10 ppm dose chlorine, using 5 gallons PLAIN 6% (chk %) household bleach to your pool. You probably should buy 20 gallons.
Also, get a cheap OTO test kit, when you are buying the bleach. You'll need to order a K-1515, to make up for the K-2005 inability to measure higher levels of chlorine, but in the meantime, simply keep your chlorine in the DARK yellow to yellow-orange range -- this is NOT a color shown on the tester.
Continue adding 10 ppm doses EACH evening. Brush the pool carefully immediately after the 2nd 10 ppm dose. Add 2 more gallons after brushing.
Amazon link => Taylor K-1515
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