I somehow JUST noticed these thin little brownish black lines all under my substrate. I watched for a bit and they’re moving! Aquarium approx 2.5 months old.
they are a detritus worm I think, harmless they live in the gravel ive got them in one of my shrimp tanks i can see but no doubt they're in them all, you can do a gravel vac to remove them also less feeding will reduce them