Camallanus, a parasitic nematode

Camallanus (Camallanus cotti). Camallanus is a parasitic nematode that anchors itself in the host fishes' intestine. Often with Camallanus your first view of the parasite may be one or several reddish worms protruding from the fishes' anus when the fish is stationary; they disappear when the fish moves. The threadlike worms are actually transparent; the red tint is  from the hemaglobin in the victim's blood they have engorged. The ones you see are all females, which can get to ten millimeters in length; the little males are a third their length.
 
At this point the host's intestine is packed with parasites, and the creatures are shedding their microscopic larval young into the water. Most species of Camallanus and its close kin shed eggs, and their life cycle needs an intermediate host, often a copepod or perhaps a cladoceran (such as daphnia); their reproduction gets disrupted in the aquarium, though copepods are everywhere, especially in planted tanks. But Camallanus cotti and the less-common C. lacustris are successful in the aquarium by dropping live young: their larvae develop within the adult female worm, who sheds them into the water; several successive generations can infect aquarium fish. The young worms are as likely to be eaten by a copepod as by another fish, but either way they get passed to the next fish host.
 
Parasitic nematodes weaken the host; what kills it usually are secondary infections. In retrospect, you may realize that the victim had been showing some inflammation in the vent area and might have been passing whitish, mucusy feces. Too often we let symptoms like these pass unnoticed. All kinds of fishes are susceptible; even a few Camallanus may overwhelm a small barb or tetra.
 
Camallanus cotti was first described in Japan in 1927, but has been distributed throughout the world, largely from the fish farms of Singapore and Malaysia, especially after 1980. At the turn of the century it was still uncommon; today it is increasingly widespread.
 
Treating for Camallanus.  Don't try to net the fish and pull off the worms with a tweezer; they are deeply embedded and you'll just tear the intestine wall. In the severest cases maybe the best thing you can do is net out the sufferer, gently euthanise it, and concentrate on the other fishes that are infested but not so far gone. 
 
Reinfection occures when fish ingest the Camallanus  from the substrate, so treatment has to be undertaken right in the aquarium that is infested with Camallanus: the whole aquarium is infested, not merely the fish victims. It might be a good idea to proactively de-worm all new fish with Praziquantel during quarantine, as a matter of course. At the end of a successful treatment, water changes will not be enough to rid yourself of this scourge: rocks and substrate should be pasteurized by a brief boiling, and plants need to be discarded or grown for a month or two in a fishless tank.
 
Chemist/killifish breeder Charles H. Harrison described the pest and its successful treatment in the Journal of the American Killifish Association, March/Apr 1998, p 57; it was archived at the AKA site but I can only find it mirrored here. 
 
Beside Praziquantel there are two other  effective drugs  to eliminate these intestinal nematode. One is levamisole hydrochloride (Janssen-Ortho). You need to bring your pH down to 7.0 or lower for this drug to be effective. In 1996 Ken Laidlaw devised the successful treatment using a 7.5% solution at 1.5 ml/7.5 liters. One commercial form of levamisole, for de-worming pigs and sheep, is marketed as "Levacide." Some useful notes about successfully treating for Camallanus with a form of levamisole hydrochloride called "Tramisole," a sheep de-wormer, are in an Apistogramma-Mailing List thread that began 15 Dec 2000, archived at The Krib. Be sure to follow this thread to its conclusion.
 
The other is fenbendazole (marketed as Panacur) is the other most-recommended de-wormer, which proved effective for Koran Weston after several alternatives were tried over a period of many months. You soak flake feed or freeze-dried daphnia in a solution and feed them to the fish. It's a good idea to give the fish a fast-day first, to encourage them not to spit it out. Fresh garlic extracts, as used to combat intestinal parasites like Capillaria, may be effective for Camallanus.
 
Concurat and Flubendazole were recommended by Dieter Untergasser (Handbook of Fish Diseases, T.F.H. 1989).
 
Link. Dr Neale Monk's succinct description of Camallanus and its life cycle.