We have been fighting with brown metal stains all summer. I did the ascorbic acid test and found it was metal related so I bought the stuff from the pool store that made it look absolutely beautiful…that is until we add the chlorine back, at which time the stain immediately returns. We back washed etc before adding back chlorine and have done the process twice with same results each time. I fear our water has too much metal as I get the same stain in my toilets when I use the bleach tablets in the tank. I have to continually add our water as something seems to be taking the level down each week. Probably some slow leak.. Any suggestions on how to get rid of the metal?
The best approach would probably be a pre-filter that can connect to your hose and take out the metals before they are introduced into the swimming pool since you are constantly topping it off. There are a couple different brands and they are a little costly, but one should last several months for topping off the pool as they were designed to filter several thousand gallons at start up.