Why is my CYA so high after winter?

[Briandpeterson]

I have had this 15' by 52" (5000 gal) steel above ground pool (overlap liner) for about 3 years. This is my 4th spring start.

All the prior years my CYA would be zero starting out in spring. Winterizing routine is a cheap winter cover and lower water below return. I usually use liquid chlorine to slam level when I close for the winter. In the Spring chlorine is zero.

My CYA is 50. I have tested it multiple times. I even bought the refill kit to ensure it was not wrong. I also filled my tube with just pool water. Totally clear.

The reason I asked is I am battling green alage. I was slamming but I was treating CYA as zero. Obviously now I know it's supposed to be slammed with 50 cya.

Simple question why this year is cya still around?

 

[Briandpeterson]

Also I have a pool cover on it now. So I figured cya as zero with UV would not matter

 

[Texas Splash]

CYA can degrade for reasons other than just the sun. Hard to know exactly why it disappeared previously. Some wacky things can happen over the winter and you're not the only one to experience it. Good thing is you're on top if it and caught the CYA change so you can SLAM properly.