I'll let Pooldoc or one of the SWCG experts give you that answer, but I believe the recommended chlorine level is 5% of your CYA level. So with a CYA of 80, I think you need to keep your chlorine around 4 ppm.

I don't know what to tell you about the black spots..but I will see if Pooldoc can take another look at the thread, and maybe he can offer a suggestion.

Janet