HELP! I have a few black spots on my plaster that look like drops of black oil. I'm 99.9% sure I had nothing drip into my pool. I've had a couple of these for months that I ignored but now I have several that I think developed over the weekend. Is this common? Do you have any idea what it is? I've had the stains that come up with Vitamin C, but this is not that.
Salt 3300 ppm
Pump runs 10 hours/day with SWG at 50%