FC level not dropping after several days

I have an unusual problem where our pool doesn't seem to be losing chlorine at any rate.

We just moved into a home with a pool. An initial test showed CYA was 140 (I use a brand new Taylor K-2006C) and FC was at 4.5 😬. I do know the previous owner exclusively used pucks, so I was expecting that. After a few days, we had small spots of algae appear at the bottom on the pool.

I raised the FC level to about 35 and kept it there with liquid chlorine and a little CalHypo, and after a few days it mostly cleared up. I know it's not quite shock level for the CYA, but I didn't have enough chlorine on hand to raise it any further.

It's now been 5 days, and I've been testing daily. The FC level has hovered around 30, and doesn't seem to be going anywhere. In the meantime I've been skimming leaves, brushing and vacuuming almost daily. The pool gets a lot of sun, though the water temperature is about 69 as the cold weather is coming in.

Despite some rain and refilling about an inch of water in the last few days (due to a leak in the offline chlorinator hose), the FC hasn't moved. The chlorinator is off, and hasn't had pucks for at least 2 weeks.

My latest test results (Taylor K-2006C):
FC 30
CC < .5
CYA 90 (somehow that dropped)
pH 7.5
TA 100

Is it just the colder water, or lack of swimming causing this? Residual chlorine from the chlorinator? My worry is I haven't added chlorine in 5 days and I may be getting a false reading or something. I've been using the 10ml DPD-FAS test to save on reagent, but other following the instructions to the letter. Any help appreciated!
 

HermanTX

Gold Supporter
May 20, 2020
2,005
Katy TX
Pool Size
14600
Surface
Plaster
Chlorine
Liquid Chlorine
I see you are using Taylor 2006 test kit. My understanding of the CL testing is if you use 25ml of pool water for CL test then you multiply the # drops by 0.2 but if you use 10ml then multiply by 0.5. Is this what you are doing?

Also, do not understand the drop in CYA from 140 to 90 if you did not drain pool. That is pretty significant change in a short timeframe assuming you did both tests and none done by pool store. Have you doubled checked your test on this?
 

Dirk

Gold Supporter
TFP Guide
Nov 12, 2017
8,624
Central California
Pool Size
12300
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Dear Bubble Trousers,

(Sorry, couldn't resist.) You could do a little "blind study" to help you troubleshoot this. Without overseeing, have another household member do the suite of tests, reading for themselves the directions. See how their results compare. This might help rule out any testing procedure errors you've been inadvertently applying. I've been testing for years, and just last month I accidentally used a 10ml sample for a 25ml test! Though I admit, I do need constant supervision...
 
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if you use 10ml then multiply by 0.5
Yes, that's what I've been doing. Actually, I've been using a 5ml sample where each drop = 1 FC. Consistently getting around 30 drops.

The CYA test may have been inaccurate at the start, as it measured over 100, so I guessed around 140. Then I performed the diluted test a few days later and it came out to 90
 
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