If you have MINOR stains use metal free after you have slowly brought down your pH with muriatic acid to 7.2 (over a few days) - no more than 16 oz acid per day per 10,000 gals. You will notice a larger usage of chlorine, so it is best to drop the chlorine down low and add Algaecide 60 (not the fifty or below algaecide as this normally contains metals) to control any potential algae bloom.

If you have MAJOR stains go the Ascorbic Acid route (do a search for this). Drop the pH to 7.2 in a vinyl pool and 7.0 in a concrete/gunite pool. I dropped mine to 6.0 but I would NOT recommend this. Use 1 1/2 to 2 lbs per 10,000 gals of ascorbic acid depending on how bad the stains are. Completely draining afterward is recommended to flush out all the metals.

Mbar is an expert on this so look up her replies.