Blackish/Purple Stains on New Plaster Pool?????

[dogette]

Hello: I need some help. I own a brand new Anthony Sylvan Plaster Pool. Overall we are very pleased with the pool. However, right after (within 3 days) the pool was filled by a local company small black spots started appearing mostly in one area of the shallow end. And one large stain that looks like a smudge mark on the step. Over the past 2 months they have multiplied. Still mostly in one corner of the shallow end on the floor. But they are throughout the pool in other areas including the walls of the deep end. Pool water is perfect except for a very high level of phosphate that was just detected. Its was 2500. We are treating the phosphate problem but these stains are still there. I tried a vitamin C tab to see if they were stains. Nothing happened. I scratch at them to see maybe if they are algea, nothing flakes off. Somewhere I read it could be maganese?? Or maybe Im thinking the plaster had something (or lack of ) in it. But why mostly in 1 area??? Any suggestions??

Also, does anyone know of a good product to clean the tiles.

Thanks,
ette