Worm?

[acowens2000]

I found 2 of these dark colored worms swimming at the top of my tank tonight after lights went out. One escaped through my net. It’s a wiggly little swimmer and I was just wondering if anyone could ID it and if it’s harmful or not.
Thanks!

 

[acowens2000]

I actually just stumbled across a post with a bristleworm on it and it actually looked very similar. It looks like tiny legs when it swims that aren’t visible in my picture and it reminded me of a centipede when I first saw it.

 

[Craigdoc]

It sounds like an epitoke of a worm, if its swimming after lights out then it's almost certainly that.

The epitoke is a reproductive organ of a worm, the worm basically breaks in half and the tail section of the worm makes its way to the surface to release sperm or eggs and other epitoke join it to basically make baby worms.

Most probably came from bristle worms like you have said.