Questions about effects of dilution due to rain


Gold Supporter
Feb 19, 2019
Orlando, FL

I'm a relatively new member to the forum and have been maintaining a high CYA pool as described here in my introduction post: First time posting

My pool is still amazing (thank you!), but I'm encountering some results that I can't fully explain. As I stated in my intro post, my strategy has been to lower my CYA level over time via draining my pool after the frequent heavy rains we have here in Central Florida, while maintaining a higher than usual FC level to account for it. So far so good in that respect... my pool has been sparkling clear and I haven't ever let it get close to the minimum FC level.

The strange results are as follows:

As of May 10th, I had the following test results:

TA: 120
CH: 460
CYA: 180

Since then, I've drained approximately 10-12 inches of water from my pool following heavy rains. I have never needed to add water from the hose. Not sure how to estimate evaporation loss, as it typically rains 4-5 days a week at this time.

My current test results are as follows:

FC 10
CC .5
pH 7.7
TA 110
CH 410
CYA 90
CSI .24
Temp 79

So, with the rain dilution that I've experienced, the TA and CH have come down about 8-10%, which seems to roughly track with the amount of rain. However, the CYA has dropped 50% during that time! An entire 90 ppm in just 2.5 months or so.

My understanding from researching posts on the forum was that CYA will degrade slowly in intense sun and high water temps, but this seems way higher than I would have anticipated. Is there anything about my test results that indicate an anomaly? Totally happy to just shut up and swim if I'm being too analytical here.

As always, thank you so much for your help and guidance!