The same thing happened to me last year. Ascorbic acid is good for the light all over type of stains from high metal water but I found it does not work well for deep spots caused by fertilizer or small metal objects.
For spot stains, I use either a very fine (>200 grit) wet/dry sand paper or a device called the pool eraser which is basically a hard piece of rubber that fits on the end of a pool pole. You can find them at most pool stores. You basically rub the stain out but don't worry it will not harm the finish. It is not very practical for large areas but it works well for small stubborn stains.
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