Falling CYA level

goofiness

Bronze Supporter
LifeTime Supporter
Jul 3, 2011
75
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.

Thanks.

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

goofiness

Bronze Supporter
LifeTime Supporter
Jul 3, 2011
75
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.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
36,294
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
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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