Ok, here's where Waterbear and mbar come in.. because I'm not sure totally if your stain is organic or metal... I'm thinking organic because you just did battle with algae and the chlorine tab did lighten it, but the grey stain sounds like metal.

I know for organic staining you need to raise your chlorine level very high.. much higher than 8ppm.. and keep it there a couple of days for the stains to be removed. Exactly how high I will have someone else answer because I don't want to give you bad information. I know at least 10ppm but it may be much higher than that.

The thing is, if you have metals in your water.. the high chlorine levels will cause them to make more stains

So... I want to ask those more experienced than I to help you now. I was actually hoping you "just" had metals and I could help you through the Ascorbic Acid treatment that I just finished up.

Evan.. Marie... can you help?