Rivulets of black stuff

[belgique]

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

Chlorine: 4
Ph 7.3
CYA 70
TA 140

 

[duraleigh]

Any pictures?

 

[Bama Rambler]

Add a little DE to that filter and give it a couple of days and see if that helps a little. I bet it is dust.

 

[belgique]

Add a little DE to that filter and give it a couple of days and see if that helps a little. I bet it is dust.

That's what i meant to try...I'm a geezer and forgot this.

Will report back.