Parasites

[Latimeria]

They sound like Oligochaetes, they are usually a sign of slight overfeeding or an inefficient filter. Fish which are messy feeders tend to create ideal conditions for these, the particles fall in amongst the substrate and act as a nutrient source for them.
The chicken liver would probably be the foodstuff most likely to create these conditions, I stopped feeding it to anything years ago for this reason, one good chew and you have liver soup.

 

[Connor0729]

planoria. they are there to clean up un-eaten food. i had them before, but some of my fish ate them, and i stopped feeding so much, so they dissapeared. they aren't bad for your fish, they just don't look so hot in your aquarium

 

[Latimeria]

Planaria are flattened (this is why the common name is flatworms), not thread-like. Here a much-magnified image of one http://www.cvgs.k12.va.us/RESEARCH/Final/sresch03/conroy/redone%20planaria%20copy.jpg

For a good description of aquatic oligochaetes see here http://www.eeob.iastate.edu/faculty/DrewesC/htdocs/ILL-p9.JPG