I am trying to use citric acid to remove brown stains from my pool. I have some questions. I have easy access to citric acid via my work. I can buy it very cheap, so I figured I give it a try. I still had chlorine in my pool when I started but since I have plenty of citric acid I figured I would start the process. My question is I added 10 lbs to 16,000 gallons of water. I still have stains and I still have chlorine showing ~0.5 ppm. The stains look a little lighter but I am not sure if that is wishful thinking. My question is should I continue to add more citric until the chlorine is zero or should the stains be gone? It has been circulating all night. I now am wondering if the stains are not metal. Also the stains have been there most of the winter so I am not sure if they are just really set into the plaster.

inground plaster pool with SWG current readings are
pH <6.8
chlorine 0.5ppm
TA 40
CYA ~60 ppm

Thoughts or suggestions? Also my Hayward vacuum is very brown stained as well. Will this treatment help remove the stains from this as well?