Need help with stains

[Vinceflat]

I have a 20 yr old vinyl Ingrd pool 21 x44. Chlorine. I changed out the htr. 4 yrs ago and started getting yellow stain on plastic steps, returns etc. Now vinyl liner.
I can remove the stains with the usual products and have added metal removers also. Chem is perfect. It's driving me crazy, help.

 

[Mendy48]

Without posting some test results and pictures, we can only take a guess. This could be an organic thing; that would be my guess (copper?). Could you post some numbers here...test results for the below components. Also, what other chemicals are you putting in your pool? No algaecide because that is a major contributor to copper. Please add a signature to your profile that describes the type of pool you have.

FC - Free Chlorine
CC - Combined Chlorine
PH - Acidity/Basicity
TA - Total Alkalinity
CH - Calcium Hardness
CYA - Cyanuric Acid
ABC’s of Pool Water Chemistry

If you don't have a test kit, I recommend the TF100 from TFTestKits.net or the Taylor K-2006C.