Few spots that won’t go away

[Hwmonkey]

My SWG hit its service cycle and was not detected for a couple days. Chloride was low as a result (0.7). I added some liquid chlorine and brought it up. Noticed a couple (4-5) of small spots on the bottom of the pool and started increasing the brushing with the nylon brush. When it didn’t improve, started with the stainless brush. 1 spit went away and pool wall suffered minor discoloration from the metal brush. I kept at it. After several weeks of this (and continued use) I decided to close the pool for activities and shock the pool. During the shock, I lowered the ph to 6.7. Brushed several times per day, kept the chlorine high ( off my liquid test kit) range and ran the pump. After 3 days, there was still no change. I brought the chemistry back to normal (7.1 ph and 3 chlorine). These pictures are zoomed so you can see it. The discoloration around the spots seems to be from the stainless brush.


Ideas on how to remedy?

thanks!

 

[kadavis]

Take a Chlorine tablet to the spots if this is black algae they will disappear, the larger spot in first picture looks more like a rust spot which could be a bleed through from the rebar or tie wire