I am trying to help out a friend with his pool. He has had the pool for 10 years and it has been a mess from the day one. The installer put a filter in that was too small so the filter ran 24/7 and you could never see the bottom of the pool. We replaced the filter with a DE and that problem was solved. Now the biggest issue is yellow/brown staining. I have tested the water myself and had it tested for metal and the results are always negative. Meanwhile i can't keep the pool from staining. Ascorbic acid always removes the staining but it always returns. I have used HTH metal control and if i put 4 qts in the pool, the staining goes away and remains gone for about a week but eventually returns.
I test the water twice a week. It seems that when the Ph falls down to 7.3 or below the staining starts. The pool has a heater with a metal heat exchanger but i put a bypass on it so that can't be a source. The only other things i can think of is the pool ladder or possibly a section of piping that was done with metal that is underground ( i wouldn't put anything past the installer of this pool ). Im wondering if there is another metal that can be in the water besides the iron or copper i am testing for.
Additionally keeping th Ph and Chlorine levels up in this pool is nearly impossible. CYA levels are good but the pool has a chlorinator with 15 tablets and is set on the highest setting. Meanwhile I can't maintain any measurable chlorine level. I have even placed tabs in the skimmer. The chlorine is dissolving but I still always need to add a powdered chlorine to pull up the chlorine levels.