Interesting. This was something that came with the pool, so it's probably old, I'll get rid of it, and get HEDP. We have a few bottles of this and that, that came with the pool. Everything that was from Leslie's Pool Supply did not have the actual chemical name on the bottle. Not one.
I had to look that info up on the web and found their MSDS sheet on a page not even connected with Leslie Pools. I think it's rather odd to package chemicals and not have the active ingredient anywhere on the label. Don't think I'll buy from them myself.

Thanks for the heads up on the sequential nature of the stain removal process. I will be careful. I set a vitamin C table on the worst stair stain, as I'd read it can be a good test to see if it's iron stain, and yep, the stain in that spot cleared up under the tablet. So, those stains probably are iron.

Interestingly, I have a koi pond also, and the best product for algae removal without hurting the fish or plants is the same ingredient as in the Polyquat, except at 4.5% strength. Works really well in fish ponds, as copper in any amount is toxic to fish, so good products can have lots of applications I guess.

Well, thanks for all the help. I'll post again when the test kit arrives.