Darkening of pool floor-deep end

Definitely iron.

I do not see turquoise, which would be copper.

In any case, I believe that it is probably copper stains.

You can try Jack’s #2 and then drain and refill if it is safe to drain without floating the pool out of the ground.

You can try a zero TA treatment, but that will eat some plaster and make the plaster rougher, but not as bad as an acid wash.
 

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Another option is to sand with a power sander and diamond grit pads.

I would probably do a Jack’s #2 followed by a drain and sand with fine grit diamond pads.

Note that draining always involves risk and you need to be sure that the pool will not float or have any other adverse effects.
 
I would read these VERY carefully...decide what you want to do...then post your plan here before you do anything...

 
Basically - lift the stains, drain the pool, refill with metal free water … and make sure there are no copper products ever used in your pool.
 
Another option is to sand with a power sander and diamond grit pads.

I would probably do a Jack’s #2 followed by a drain and sand with fine grit diamond pads.

Note that draining always involves risk and you need to be sure that the pool will not float or have any other adverse effects.
 

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