Detritus worms
Annelids “Segmented Worms” - Including:
DETRITUS WORMS
The annelids are the phylum of segmented worms which includes earthworms. Most annelids are NOT purely aquatic in fresh water; the annelids found in freshwater are all oligochaetes (which means "few-bristled") and are not very important in the freshwater ecology as are their marine cousins, the polychaetes (which means "multi-bristled"), are in marine environments.
All the oligochaete worms are hermaphrodites (an organism that posses both male and female genitalia). Many are nearly microscopic too
Detritus Worms (from the group of worms called Oligochaetes, sub group naidid worms) are often misidentified in the aquarium hobby as Planaria without close inspection by many internet articles such as about.com.
Many of these worms are accidentally introduced by live plants, gravel (especially in the case of common Detritus Worms) live fish foods, and even brought in with fish transfers.
(Click picture to enlarge)
Many not familiar with Detritus worms will label these as everything form midge larvae, Planaria to baby earthworms, of which none is true. They are very common and most often seen during vacuuming and other cleaning procedures.
These worms generally are not a problem, however high numbers of particular species can indicate low oxygen levels and low filter productivity, which very often suggests some degree of pollution caused by poor cleaning procedures, over crowding, over feeding and poor filtration
Please reference these sources for even more identification (and further information so as to dispel the internet aquarium myth that these are Planaria):
Planaria
Aquatic Life: Worms ,
Oligochaeta Worms
Aquatic Worms
