I'm also going to guess not enough HEDP. Some years ago (around 2006?), before I was aware of this site, I had staining issues on my steps and vinyl liner. My pool store suggested using ascorbic acid to remove the stains then use Jack's Magic Purple Stuff to prevent them from coming back. They told me the metals probably came from the small amount of impurities in the pool salt (I'm not sure of this). I also contacted Jack's Magic also for guidence and info. Jack's recommends you keep HEDP at a certain level to prevent staining, so I ordered a sequest test kit so I would know how much I had in my pool.

The ascorbic acid worked well to remove the stains so I started adding HEDP and testing the sequest levels. Apparently the metal levels were quite high as I kept having to add more HEDP to maintain the recommended levels. Jack's said to keep adding the HEDP until the levels stabilized in the recommended range. I think I went through 1 or 2 cases (!) of HEDP before the levels finally stabilized. Since then I've been adding a maintenance dose each week.

Fortunately the stains did not return during the initial treatment and I've not had any stains since.

Earlier this year I had asked the question about whether or not I still needed to keep adding the maintenance dose of HEDP (Jack's told me their product would allow the metals to be filtered out, but information here seems to indicate that HEDP just keeps the metals in suspension). Ben had me do a bucket test to see if I still had metals, and it appears I do not (where did they go?). So the past month or two I've only been adding half as much HEDP and so far no staining. I may reduce the amount again and see what happens (or not, I really don't want to deal with stains again!).

Hope something in this story is helpful to you.