Yes, I've asked variants of this over the years but it persists: I keep getting rivulets of something black on the bottom. We're not in a dusty area so I don't think it is dirt. The new SWG is keeping Chlorine great so I don't know how it could be algae esp with this data point: yesterday I vac'd to waste....perfectly clean bottom. In 90 minutes (!!!) the rivulets were back! The water is crystal clear sparkling. In past days I have vac'd using skimmer socks and don't find much.
Ideas? Thanks, Steve