Black marks on pool steps

[waterbear]

I'd like to see a pic. Also, be aware that the magic eraser is a actually an abrasive pad and can damage the gel coat on your pool and make matters worse. Black stains are usually either copper or mangansese or an algae. Start with a full set of water test results done with your ColorQ (which, btw, while good, does have some limiitations compared to the the LaMotte 7022 FAS-DPD test or a Taylor K-2006)