Removing rust (?) stains?

[limdoan]

Hi I have this rust (?) spot/stain in the attached spa. I read that you can use vitamin C tablet (ascorbic acid) to clean this. Is this ok?
Secondary question: What caused this?
Thanks!

 

[reggiehammond]

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The rust is either coming up from the rebar below (worst case scenario) or is a surface stain from something that sat on the surface and left behind a mark.

 

[1poolman1]

Hi I have this rust (?) spot/stain in the attached spa. I read that you can use vitamin C tablet (ascorbic acid) to clean this. Is this ok?
Secondary question: What caused this?
Thanks!

The large one looks like a penny, they're no longer completely copper. Does anyone have a BB gun? That's what the small one looks like.

 

[limdoan]

No BB gun. I remember there were rocks or a piece of the ... what's that called, platform surrounding the inground pool thing...
Penny... maybe. Or small kids toys...
Here's another pic.

Hope not rebar issue.