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Last year I had a localized stain (about 6'x3') downstream of the outflow where I put most of my chemicals in. I thought I had stained it by some undissolved chem's settling on the bottom, but now (this year) I have the same color ring around the top of the pool, and a couple more small areas of a similar stain. I've tried to brush it and it doesn't change appearance at all (someone mentioned Mustard would initially come off, but return the next day...this stuff doesn't change at all after brushing).

Original stain doesn't seem to be "growing" per-se, just appearing in different areas.

Any ideas? My one checmistry issue is high stabilizer (CYA about 90), so I know my shock level needs to be higher. Using Ben's best guess pool chart, I shock at around 25, and was able to maintain this level for many days this season (I had to open the pool early due to some yard construction, and it was way too cold to swim, so I let it stay in shock mode for quite awhile).

Water is otherwise clear, virtually no CC.

Typical day's measurements:
FC 8-10
CC < .5
TA 40-50
CH ??
CYA 90
PH 7.4-7.6 (keeps dropping maybe due to Acidic NJ rain?) Also trying to elevate the TA to settle the swings.

I use mostly BBB method, but use trichlor tabs for maintenance since I'm out of town a lot.

Vinyl liner
30x15 above ground
Swim pro sand filter
Hayward 1.5 HP power flo matrix pump
Using the recommended test kit