The general rule is: never use a pool chemical you don't need.

Phosphonic acid -- HEDP -- can be used to lift copper stains in the early stages. But it's effect is temporary, because it breaks down to phosphates (algae food!). I think Chem_Geek can give you some more detailed technical info.

But, the bottom line is: don't use it, if you don't need it. If you DO needed (high copper), you need to fix the cause of the problem (dissolving copper OR adding copper algicides). Then remove the copper from your pool and stop using phosphonates.

If you want to improve the 'feel' of your water, add borax.

Ben