Cya levels getting lower with time

[Bilben1]

Hi, I'm new to this forum. I have a 25000 gal Gunite plaster pool. The pool is over thirty years old and is still in good shape. However, after a drain and acid wash 2 summers back my son only used 3 inch Leslie's pucks to maintain the fc using the std oto test, but sure enough we started having algae problems last summer. I had the water tested and the cya level was over 100. So using the info supplied here I just stopped using pucks and just used 10% bleach instead slamming the pool and it did work got the algae under control. Ive been using bleach and acid to maintain very high levels since late last summer.

I was planning to drain the pool to make some minor plaster repairs and get the cya level to about 30 or 40 after refill and then use the bbb method for maintenance. I took a sample of water to leslies and when he tested the cya was back down to 60. I was very surprised. I didn't know that the cya salt could breakdown
The sales guy said he was told that uv does break it down with time. Does anyone know if this Is a fact or will draining the pool still be necessary.

Thanks for your hep in advance.

 

[JasonLion]

Welcome to TFP!

CYA does break down very slowly, in most cases too slowly to be of any real use. A typical pool might lose 20 ppm of CYA a year. There are also situations where CYA can vanish much more quickly when FC is left at zero for a time, unfortunately that can't be done on demand.

 

[Patrick_B]

Welcome to the forum. Glad you joined up.

Cya does go down. Duilution with water replacment is one cause. There is some slow breakdown from Chlorine, and there is loss if the pool gets green and stays that way for a while. I would differ on the breakdown from UV as a primary reason for Cya loss. One of it's primary functions is protecting Chlorine from UV, so it can take some beating from the Sun.

If I may kindly suggest, it would be good to have your own kit to test this yourself and be certain. Unfortunately, Cya is one of the results we see vary a lot from store to store, sometimes the same store depending on the test they use, and who runs it. You may well be 60, but it's hard to say with just one reading from the store.