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...