Chlorine only test cylinder


Well-known member
Jun 30, 2008
Suffolk County, NY
Is there any reason why after a year my chlorine only test cylinder would become very black?
It seems to have a residue on there that won't come off.

I just purchased a new one, but Im wondering if its the same reason that I get dark lines on any plastic or fiber glass at water level?

No big deal... It comes off easy enough, but it appears relatively quickly. I scrubbed it off yesterday and it was back this morning after a heavy rain...

Bama Rambler

Mod Squad
TFP Expert
Bronze Supporter
LifeTime Supporter
Jun 22, 2009
SouthWest Alabama
That's why there is a "chlorine only" vial. You can clear it up by bleaching the vial. It's caused by the DPD reagents.

The ring around your pool has nothing to do with the black around your vial. Get a scum ball and throw it in the skimmer, then clean the pool with Magic Erasers.