I have a 34,500 gal pool in FL. I was running a Total Control system with a CC-15 cell that I switched about 1 month ago to percentage & ph due to my ORP control not functioning well.

3 times now I have had 25ppm -40 ppm of CYA rapidly go to 0ppm.

Most recently I have been very stable at these readings, starting about 1 week after adding CYA

Total Control set @ 51%
FC: 5.5-6ppm, pH: 7.5, CYA: 40 ppm, TA: 40, CH: 380 ppm

Today, 5 days after my last CYA reading all those tested the same, but CYA reading (I tried 3 times) was 0 ppm. It makes no sense to me as previously, low CYA would have caused the FC to drop to 1.0-2.0ppm even with the salt system set to 100%.

I can not imagine any bacteria could have done this. What is consuming my CYA?

Any ideas out there.

What are you using to test your CYA level? The TF-100 or Taylor K-2006 test kit or are you using test strips?

There are only two ways for CYA to go away rapidly. One is massive water replacement. Sometimes when you have an autofill system there can be a leak, but the autofill system maintains the water level so you don't notice. The other way is for the FC level to fall to zero for a period of time and for the correct kind of bacteria, which consume CYA, to get going in the pool.

CYA can breakdown in other ways, but that happens quite slowly. It would take several years for CYA to go to zero because of normal breakdown.

I tested again this AM and this afternoon and everything seemed right but CYA which was not possible so I rechecked my test kit (TF-100). It turned out the R-0013 CYA Reagant had been accidentally swapped out with the R-7065 CYA Standard Solution.

As to the two prior times my CYA disappeared, it was when my Autopilot system was not working properly, and I presume the chlorine level had dropped as noted by JasonLion's post.

I apologize for wasting everyone's time and no longer need help with this issue.

Thanks.

A side discussion on bk406's CYA loss was moved to here. JasonLion