First, thank you for all the valuable information. Have been following this method for 5 years. Just installed new vinyl liner, filled with well water, used 5 gal bucket filled with batting. Have only added poly 60, brushing & filtering 24/7. Filtered loose rust out but stains wherever there are angles. Just tested with Taylor kit:
FC 0
PH 8
TA 330
CYA 0

Now I don't which to address first: start ascorbic acid treatment or lower ph & aerate to lower TA. Your advice would be greatly appreciated.

16x36 ig vinyl
Hayward 300# sand filter