Algae? Stain? with pictures

[Guest]

Recently, I noticed some brown spots on the floor of my pool. There a several of them, but not many.
They can't be brushed off, and do not grow (yet) either.
I have shocked my pool with FC15-17 last Sunday, and overnight is less than 0.5.
current:
FC: 5
CC: 0
pH: 7.6
CH: 260
CYA: 40 (just replaced 2/3 water last Saturday.)
Borate: 0 (i guess, the test strips will be delivered tomorrow)

Any ideas about what they are and how to remove them?

Thank you in advance~

 

[257WbyMag]

Looks like metal stains. Here's how you can tell. Take a vitamin C tablet and scratch it on the stain. If it lightens, it's metal staining. If it doesn't, try taking a chlorine tablet and setting it on the spot for about 20 or 30 minutes. If it goes away then, it's organic and will disappear with proper chlorination; faster if you shock your pool.

 

[Richard320]

Very good chance those are rust. Even the tiniest piece of steel can leave a mark. I had a few freckles after I had a fence replaced.

Solution: Vitamin C tablets. I just bought the cheapest store brand. I smasked them into about quarters and stuck them in a nylon with some smooth rocks and tied a string to it. Then I set it on the spot. It might take some nudging with the pool brush to get it on the right spot. I just left it there overnight. Done.

 

[Guest]

thank you so much for you quick reply!
I will try Vitamin C, and report results tomorrow