In previous years, I was getting a yellowish/brownish stain on my skimmer and pump baskets, returns and a little on my white stairs. The pool builder installed a DuoClear system (pool built in 2007). Prior to finding Trouble Free Pool, I used to toss in a plastic ball filled with some sort of liquid that the pool store sold and the stains would disappear, but a few months later they'd be back (probably ascorbic acid).
Last fall, I removed the mineral 'bag' from the mineral cartridge (with the DuoClear, the cartridge has to be in for the system to hold water), reinstalled the cartridge (sans mineral bag) and did a Vitamin C cleanup. So far this year, after one month of running (plus one month last fall) there are no signs of any staining. I truly think the staining was caused by the mineral cartridge (my pool chemistry has never been way out of whack, and there are no metals in my fill water).
Just as mentioned many times, by many knowledgeable people here, those 'mineral' systems for pools are more trouble than they're worth.