Black smudge on pool walls


Well-known member
Sep 8, 2015
Dallas Texas
I have a black “smudge” on the wall of my pool in a couple of spots. It reminds me of rubber/brake residue. It comes off when I wipe it with my hand but brushing doesn’t seem to get it off. Any thoughts? Is it just dirt (lots of construction around here ) or something else?

My pool details
Chlorine is 7 (usually have to add 1-2 ppm every 24 hours but that hasn’t changed)
CYA 40.

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Sounds like a scum line we can get at times. No real rhyme or reason. We can try to blame excessive oils in the water during swimming season, but sometimes it just happens. I usually get it once or twice a year on one corner and perhaps in the skimmer. We use a little elbow grease and perhaps a Magic Eraser to remove it on ours.


TFP Expert
LifeTime Supporter
Jan 6, 2010
San Dimas, CA (LA County)
Look closely at all of your pool tools.

When one of my brushes started getting worn to nubs, I started dragging the plastic spine against the plaster leaving blue stripes. It wasn't intentional.