Hi just wonder how can I treat the pool wall stain, as shown in the attached picture. The stain cannot be brushed off. I have been running the cleaning robot 30min daily and brushing the wall weekly for a few weeks. Didn't get the condition improved.
Highly appreciate if anyone could give an idea.
Test levels today;
CYA ~80 (very subjective though with the TF100 kit)
CH 700~800 (cannot replace water due to water restriction ...)
Shared album - Eric Yang - Google Photos