For the last 2 years I have had a dark, shade-like stain only in the deep end of my fiberglass pool. Does not seem to scrub off with abrasive sponge, etc. Seems to spare the very edge of the pool corners. I theorize due to the accumulation of chemicals in the corner of the pool that "bleaches it" but this is a guess. I am not certain when it appeared in relation to the addition of chemicals. I had added an algaecide from Home Depot prior to close a couple years ago, so I postulated it maybe copper and related, but not sure as it was overwintered then.
I took a sample to the pool store to discuss with them about 2 years ago and they said the "metal" was a bit high, and I used a sequestering agent. On retest I was told it was fine.
I included a picture. Is there anything I can/should try? I suspect this is just cosmetic but it bothers me.
Thank you, and best regards to an awesome site.
FC = 3.5
pH = 7.5
TA = 150
CH = 130
CYA ~ 40
Borate = 30