I'm confused too - I don't understand why the vitamin C worked the first time, and not the second. The only other thing I can think of is that it may not be an iron stain. There is a product by "Jack's Magic" that has 3 different tests to tell you what kind of stain it is. Stains can come from copper and magnesium too. I have only dealt with the brown iron stains. I wouldn't raise my calcium any higher if I were you 350 sound good enough. I just don't know - maybe someone else can chime in with some ideas? Don't throw any more chemicals in for now. Ascorbic acid ususally works right away, even if your chemicals aren't perfectly in sync. See if anyone else can come up with some ideas.