Camallanus Cotti and Camallanus lacustris.
The camallanus worm is a type of nematode worm that can inhabit the intestines of tropical and semi-tropical fish. This worm is quickly identified by the red paintbrush like worm tails that may protude from the affected fishes anus. They are small parasitic worms that attack the intestines and colon. They attach very firmly and any attempt to pull out the worms will greatly harm the fish. It is highly contagious, the fish are infected by eating the worms. Like a few other fishy illnesses it is easily transferred from tank to tank, if one tank has it you should treat all tanks. Other signs are emaciation and a disinterest in food, after all they are intestinal parasites that steal nourishment from the host and inflame the digestive tract by their presence. This parasite can take up to 3 months to rear its nasty little head! And due to wholesalers practices it has become resistant to most de-wormers on the market. They have a 30 day lifecycle.
Treatment; Levamisole, Fenbendazole are the 2 best options. Be very very sure to vacuum up all of the discarded worms so you dont have a re-infestation, most treatments are every 2 weeks.