Water clear, losing chlorine, scratching my head

Jul 26, 2020
10
Phoenix
Pool Size
10000
Surface
Plaster
Chlorine
Liquid Chlorine
I’ve been testing and maintaining my pool the TFP way since August 1. Testing daily or twice a day. Last night, I tested FC 3.5 and ph at 7.9 I added muriatic acid and chlorine and restested 6.5 FC and 8.6 ph. TA is 100 (my fill water has high TA) Cya is 50-60 (dot disappears between them) CH is 450.
This morning, I test chlorine ad ph at 4.5 and 7.8. WTH?!?
I look at my crystal clear pool. Could something be blooming? Idk.
my baskets are clean. I had backwashed a day or two ago.

Anyway, out of precaution, I add a gallon and half of 10% chlorine (which should put me at FC of 20-21) Retest 2 hours later.
FC is at 9.5. Added another gallon of fresher chlorine (should put me at 19-20) Test 2 hours later FC is at 14.5. I decide to wait to see if it can hold before adding more. I test 2 hours later at 12.5 FC and then another 3 hours at 11.5 FC. I can’t spot anything. It is 110 degrees out. I then tested for phosphates using Taylor’s kit - 125 ppb. If I did have algae, wouldn’t my phosphate level be higher? I know the recommendation is to slam. But I want to make sense of this first before I needlessly waste money on using up my supply of chlorine.
 
Hot and sunny and much UV and CYA of 40 to 50 the sun was chomping away at your FC like a heavy lunch :) Now that the sun is down check to see what it is and check in 2 hours, i bet its the same...
 
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