Clear pool - FC still dropping - What could be going on?


Well-known member
Aug 13, 2010
Neptune City, NJ
A few days ago, I was getting >0 CC readings and was trying to figure out what to do. So I took a step back, got my other numbers in order, and let the pool sit for a day. Did nothing.

So here's what I did yesterday. Water is crystal clear, BTW. And CCs went to zero went I let it go. (I sorta wanted to see if they would burn themselves away.)

Water temp around 70 degrees. 28k gallon vinyl lined inground pool

8am - FC 0, CC 0, TA 100, CYA 50, PH 7.2 ---> Added 2 gallons of 12%

9am - FC 3, CC 0 ---> Added 2 gallons

11am - FC 7.5, CC 0 ---> Added 2 gallons

2pm - FC 9.5, CC 0 ---> Added 2 gallons

4pm - FC 13, CC 0 ----> DID NOTHING

7pm - FC 8, CC 0 -----> Added 1 gallon

8:20pm - FC 13, CC 0 -----> DID NOTHING

7am - FC 9, CC 0, TA 100, CYA 50, PH 7.2 ---> Added 1 gallon of 12%

8:30am - FC 12.5, CC 0

This is an AWFUL lot of chlorine for a crystal clear pool with no CCs. When I go outside and walk around the pool, you catch whiffs of chlorine. But if I stick my hands in the water, and gather some up, I don't smell anything from the water itself. Should there be any smell whatsoever from a perfectly maintained pool, with appropriate FC?

I did buy an aquarium ammonia test, which on the day I was letting the pool sit, gave me a reading of 4. But from what I have found, I'm not sure how valid that is.

(As a side note, I bought a large batch of chlorine that was manufactured in March, and it seems noticeably less filled and smelly, than the new batch I bought 2 days ago, manufactured in late April. So I suspect the FC rise isn't predictable due to varying potency.)

What's going on?

Thank you
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Well-known member
Aug 13, 2010
Neptune City, NJ
Target FC for CYA 50 is 6-8. Why are you adding chlorine above that? It only wastes chlorine.

Do a Overnight Chlorine Loss Test.
I put above 10 to see if maybe there was an ammonia problem that could be cleared.

My tests above were my OCLT and they showed that FC was dropping overnight.

Plus, 8 gallons to get above 10ppm suggests something is strange. It should have taken 2 1/2 gallons.


May 28, 2018
Staten Island, NY
From the OCLT instructions -
“If you lost more than 1.0 ppm of FC, then there is something in the water that needs to be removed and you should SLAM, or continue SLAMing, the pool.”

so... there you go. SLAM.