I have been successfully maintaining the CYA level in my pool between 40 and 50 for several months. We keep our FC levels a little high because of a hx of black algae. This has worked well for several months.
So I had stopped testing the CYA level frequently since it had held steady and I have NO sources of CYA adding to the load. I use liquid chlorine only.
Recently we had a series of thunderstorms that added maybe 1" or 2" at most to the pool. I didn't think to check the CYA level afterwards because a slightly lower CYA level would be fine. It's higher I don't want to do. Did have to adjust the pH down a little more than usual.
So about 5 days after the rain we notice a resurgence of the black algae. Very small spots but definitely an increase in growth.
I raised the chlorine level up to 30 for 3 days (FC stayed steady overnight) and did heavy brushing, etc. and the algae retreated but then resurged again once I dropped the FC back down to 8.
So I just checked my CYA level (had to go buy more reagent) and it's up to 90!!!
The ONLY thing that could have done this was the rain. But I thought rain diluted CYA not increased it.
Anyway, I am doing a partial drain and refill but was wondering if anyone else had seen this.
After refill I will recheck and adjust numbers as needed. Probably drop the pH to 7.2 and increase FC again.
Plaster pool 17000 gal, Hayward pumps w/4 cart filter, Jet-vac sweep