From The Aquarium Wiki
About Capillaria Worms
Capillaria worms are a type of Roundworm (Nematodes). They are thin hair-like, 0.5-2cm (0.2-0.8") long, parasites that live within the gut of various species of fish like Discus and Angelfish and reproduce by depositing their eggs within the faeces.
The affect of the worms on the individual fish can cause lose of appetite and emaciation thus leading to sudden death.
Fish such as Discus can darken in colour and infected fish faeces will be very thin and translucent.
Special medications are required to get rid of this pest.