Overnight FC loss confirm something?

It seems you have it under control. When I needed to clean my filter I had never done it and have a DE filter. I googled and found a few videos and watched them. Got it all taken apart and back together with a few issues leaing around the seals. Sealed them off better and all was well. I was afraid just like you, but once I started I was good to go.
 
So many times I read posts that end when the problem ends...and I think, hmm...wonder what happened the next couple days. So, here ya go.

Monday 1045pm: FC 21.5
Tuesday 6am: FC 21.5, passed the ONFCL test
Tuesday 930pm: FC 16.5 (lost 5.0 from 6a to 930p)
Wednesday 9:00 pm: FC 12.0 (lost 3.5 in 24 hrs) Much better than before this episode.
Thursday 930pm: FC 8.5 (-3.5 over 24 hrs again) brought up to 9

So I haven't added any chlorine since 5pm on Monday until Thursday night. That's been a nice change from the previous 5 days...
I'm still getting SOME black gunk, but nothing like that last pic. When I check the skimmer I sweep the waterline with a Miracle Eraser.

If you've followed along, thank you for sticking with me. If you are learning something, glad I could help. :wave:
 
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