I`ve a deep 25K gallons inground pool (fiberglass walls with concrete bottom).
During the winter the alkalinity went up and some metal stained the walls (curiously, only the walls, not the bottom). Plenty of brown spots.
It is probably copper since 1 year ago I had algae three times and killed it with few quarts of copper based algaecide... Last year in NC there was an invasion of yellow algae. I first checked for black algae but rubbing with chlorine tablets did not change the situation
Then, I bought some ascorbic acid tablet (2.99$ vitamin C) at the local grocery, and rubbed it on a couple of spots. Amazing.
The ascorbic tablets work like erasers on pencils drawings!
Yesterday, I bought 36 lbs of citric acid (it is very cheap, I paid 105$ with shipping), and I will follow the AA instructions. Once the metal is back in solution (if it does not come out, I`ll jump in and rub all the spots with the acid powder and a sock), I plan to drain 2/3 of the pool and refill it. In this way I reduce the CYA (80) and the alkalinity which I kept high to protect chlorine from fluctuating too much under the sun (I probably exaggerated, and that`s the reason the pH went up to 7.8-7.9 and the metal solubility in the water decreased, now the pH is around 7.0 and it will be like that until the end of the citric acid treatment so the metal gets back in solution more easily).
Regarding draining the pool: some pools can be drained, others can not. It depends on the structure, construction, water table level, position on the yard. My pool can be drained. Four years ago I kept it empty for 1 month while I was patching/sanding fiberglass and painting with epoxy paint. No problems whatsoever. If stains were hard to remove, it would be cheaper for me to drain, rub with ascorbic acid, and respray with a thin coat of epoxy paint.
I will post updates soon.