Falling CYA level


Bronze Supporter
LifeTime Supporter
Jul 3, 2011
Stockton, CA
I've been adding gallons of 10% bleach this summer, per PoolMath calculator, to prevent a recurrence of yellow/mustard algae this spring. I finally decided to test the CYA (using all fresh TFT reagents). I was shocked to find it somewhere between 1-15, confirmed by multiple water samples, and two testers.

In April the CYA was 38; I added 32 oz liquid stabilizer and got it to 42; not tested since. In May I had my usual episode of mustard algae in streaks on the north facing pool wall. I got the pH to 7.5, and SLAMed to mustard levels for several days. Actually, I was way above, with FC around 22-25 for several days. Passed the OCLT, no CC. Since have tried to maintain Min FC of 6, Target 8-9. But, the pool in full summer sun, with only me swimming, is dropping from 8>4.5 during day, and I'm adding a huge volume of 10 % bleach every night. In past summers, the FC drop in daytime would be around 25%; now it's more like 40%. This summer, I'm maintaing mustard levels of FC; I did not do this in the past, and had recurrent episodes.

I just found an old thread about dropping CYA levels. Mine is probably related to the very high FC levels during SLAM back in May. I had never thought that CYA ever went away, except with pool flushing.

Any other insights or advice, other than to add some stabilizer now.


Recent numbers:
pH: 7.5-7.8
FC: 4.5-9.0
CC: 0-0.5
TA: 70
CH: 340
CYA: 0-15???


Bronze Supporter
LifeTime Supporter
Jul 3, 2011
Stockton, CA
Thanks for the reference on CYA. We are in Stockton, CA, and have had several 100+ days, but the pool is usually 82-85. I swim laps, but our 5 y/o grandson is in on weekends, so there is little splash loss. No leaks as far as I know. I never let the FC get below 4; I kept the FC level 2-4 all winter. The water is very clear.

As I mentioned, there have been episodes of yellow streaks and smears on the N. facing wall. I was in denial that it was mustard algae. I used to brush vigorously, and sometimes used a pumice stone. Finally, this spring I decided to treat it, and did the mustard SLAM. Touch wood, no more mustard. So, I think it was the high FC during SLAM that used up the CYA, and I will add more now, up to around 40.


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Test the CYA every month. Elevated FC levels have little effect on CYA levels. UV is the primary oxidizer of it.
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