Green soluble iron eventually is converted to orange-brown insoluble iron that shows up as stains, orange-brown filter debris, or orange dust on the pool bottom (makes the water look green due to the optical effect of adding blue water with orange dust)
If you fill your pool with the same water you fill your toilet, than the interior of the toilet TANK will tell. But, much 'city' water has some iron. What you are looking for are dark orange or brown stains.
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