A Case of the Pool Measles

[waterbear]

From your description this could be cobalt....only seems to happen to fiberglass pools from what I understand. I will have to do a bit of research and see what I can find out about it. From my understanding oxalic acid is the most effective treatment for this. United chemicals has an oxalic acid product designed for colbat staining in fiberglass pools. Have never used it so I can't tell you how well it works. There is some data that may indicate low calcium levels might be a contributing cause of the cobalt staining but nothing conclusive that I have been able to find.