That certainly makes sense. What I find odd (and why I mentioned it) is the timing…I've been shocking my pool for a solid week now (to 30-35 FC), with nothing/no one in the pool during this time (other than my arm to retrieve water samples) so why would a black ring of goo suddenly show up on Sunday morning? I check and empty my skimmer/baskets daily and Saturday there was no line at all.

I've had dozens of (mostly) girls in the pool with very health-conscious mom's who indeed slather and spray them right before they swim…even tho I tell them repeatedly it's just going to come off in the water. I've never had anything black before. Is it the high FC levels "leeching" stuff out?