One of the people that works for the manufacturer of the coating came out and tested the water. He claims it was Maganese. The funny thing is that for a year and a half nothing showed up. I have pictures of the pool in April and there’s not one spot, suddenly all of these dark spots appeared. How can I test to tell which metal it is?Do you know what the manganese levels are in the fill water? Manganese and iron often are found together due to the minerals they are derived from so your “green” water is likely due to iron oxidation (yellow iron oxide + blue water = green hue).