My Unknown Worm

[Mickle]

Today I decided to finally look into a worm I have had in my tank, I want to figure out what exactly it is out of curiosity (if possible). I took a video of it using a USB microscope, it looks like it eats sand? I was hoping I could get to see the end of its body but it wouldnt cooperate it seems like its body goes on forever! Im counting 5 tentacles(?) on its head, Im not sure what the identification factors are for polychaete worms. The usb microscope has really bad FPS so sorry bout that.
This worm is very thin, less than 1mm

Edit: Pretty sure its a beneficial Eunicid worm judging by the sand in its body

Sand in digestion tract timestamp

Full video:

 

[vaguelyreeflike]

looks like a worm from the dorvilleidae family to narrow it down more

 

[ISpeakForTheSeas]

Im counting 5 tentacles(?) on its head, Im not sure what the identification factors are for polychaete worms.

Tentacles are an important ID factor for polychaetes; five tentacles would indicate a Eunicid worm, but the worm eating sand is a good sign that it's most likely harmless.

So, yeah, barring clear, close up pics of the head and bristles, I'd agree with your ID, OP - likely a beneficial Eunicid.