Plaster discoloration

[HungryWookie]

Hey team!
I've got an ~18,000 gal pool in-ground gunite/plaster built in 2020. It has a salt water chlorine generator do pretty well with the with water chemistry, tho I admit to being a little lazy with the muriatic acid, so my pH is commonly around 8.0. I've seen very localized deposits of scale accumulation on the side walls this last summer, but with a fingernail and staying ontop of the acid, it went away.
My problem is in seeing areas of light brown plaster discoloration on the bottom of the pool. It doesn't seem to scrub out, and it is not texturally different like the areas of scale. It doesn't look too bad, but I wanted to get ahead of it and fix it if I can.

I live in Wilmingon,NC and the pool is not heated.

Here are a couple pictures, hopefully you can see what I mean.

Thanks for any advice in advance!
-Sully

 

[PoolGate]

This may just be what they call "mottling" which tends to happen with all new plaster. Mine is similar.

 

[JamesW]

Maybe iron and possibly some copper.

Drop a vitamin C tablet on a spot.

Get the Jack's Stain ID Kit.

Stain ID Kit | Jacks Magic

jacksmagic.com

 

[JamesW]

Can you show the system?

 

[HungryWookie]

Can you show the system?

That stain ID kit seems interesting. Might try that once the water warms up. I guess I could bring my water to a local pool store and have it tested for metals?
-Sully

 

[JamesW]

Drop a vitamin C tablet on a spot.

Can you pull the cartridges and show what they look like?

Pull the cell and look for scale.

Look in the pipe after the cell for discoloration.

 

[ajw22]

I guess I could bring my water to a local pool store and have it tested for metals?

The metals will not be in the water and show on tests if they deposited on pool surfaces.

 

[HungryWookie]

cell died after 3.5 years and I replaced it about 3 months ago. The old cell was scale-free, though. I'll try the vitamin C trick and see what happens.