black spot

Scrubbed and scrubbed with stainless steel brush. Used pool rx but can't get rid of stains in our in ground concrete pool! HELP!

Hello Fellow Pool Boys and Girls, We just finished building our pool about a month ago. Unfortunately, we've developed a pea sized spot with a black center and brownish cloud moving away from the center. I tried placing a chlorine tab on the spot, for approximately a week, and brushed...