There are many pool stores in my area, all carrying different brands of stain and scale control. I can tell immediately, if the product is phosphonic acid / etidronic acid as the metal stains on my white stairs disappear within minutes as opposed to the non phosphonic (usuallly labelled phosphate free or touting 'biodegradable' on the label) where the stains remain. These ones may bind up the metal in my water, but do nothing for existing stains, and don't seem to list the active ingredients. I had been using Sani Marc 3-Met-0 and it definitely does not remove existing stains.
It was refreshing to find a brand where the bottle was clearly labelled with the active chemical. Unfortunately, this brand does not appear to be available in the US (I'm in Canada), however, there seems to be more choice of sequestrants in the US to choose from (for example, I've never been able to find any kind of Jack's up here).
The brand I found is Omni and after calling the toll free number (also conveniently located on the bottle), it appears that they also produce a similar product under the Bio Guard name.
Just a heads up for anyone specifically looking for a sequestrant that is Phosphonic Acid based.