Checking your toilet tank is a good way to see whether - and what kind - of metals may be in your fill water, provided you do NOT have a water softener, and provided that your pool and your toilet get water from the same source.
To test your pool, you can use either Iron Out (from the laundry section of Walmart or wherever -- contains sodium hydrosulfte and sodium sulfite) OR Vitamin C tabs (ascorbic acid)
If you put either one on iron or copper stains, they will lift or at least change those stains. But, they won't affect organic stains much at all.