Capillaria, another parasitic nematode worm

Capillaria are parasitic nematodes, smaller than Camallanus and proportionately very thin ("capillaria" mean "hair-like"). There are numerous species, most of which were originally from South America, where they co-evolved with certain specific hosts. A light parasite load can be sustained without apparent effect. The smaller the host, however, the more troublesome even quite minute parasites can be; Capillaria may be a cause of some inexplicable wasting-away you observe in small tetras. The fish stands away from its fellows, begins to hide, begins to look starved and eventually dies when a vital organ is no longer working.
 
When intestinal parasites get transferred to the "wrong" host, as happens in captive populations, they can do unaccustomed damage.

There are two kinds of Capillaria. Most live rather innocuously in fish intestines and cause trouble only when they become too numerous. Their eggs are shed and passed out with the feces. When an egg is ingested by another fish, the life cycle gets repeated. In the crowded conditions of the aquarium, this may happen more frequently. 
 
Other Capillarian types have alternate hosts. If your fish is the intermediate host and ingests an egg, the hatched worm bores through the intestinal wall and makes its way through muscle fiber and internal organs to encyst. There it awaits its final, or "definitive" host, the one that eats the infected fish. Encysted capillarian worms are untreatable, though they won't multiply within their intermediate host. If the cyst is near the skin, you might confuse it with a tumor or a digenetic trematode. Capillaria have been transmitted through tubifex in lab experiments, according to the ZFIN website, but the common method of transmission in aquaria is from fish directly to another fish.
 
Treatments. Treating Capillarians with Mebendazole/Trichlorfon (Fluke Tabs, from Aquarium Products, for instance). Trichlorfon is a brand name for dimethyl trichloro hydroxyethyl phosphonate, which is also the active ingredient in Clout. You'd add a second treatment after ten days, as the eggs are resistant. Bioencapsulation is a good technique for getting the medication into the fishes' intestines, where it can work.
 
Praziquantel (2mg/L) is now the recommended alternative, much less toxic to fish. 
 
Garlic cure. The famous Discus breeder Jack Wattley has developed a technique to control Capillaria nematodes in Discus using fresh-squeezed garlic in every feeding over a period of several weeks. He published his "garlic cure" in a book on Discus, but it really caught on when the technique was published in Tropical Fish Hobbyist, May 1999. A single "shot" of garlic, or the extract merely dissolved in the aquarium water, is useless. But stand by (as of the winter of 2002-3) for a wave of enthusiasm for garlic extracts to treat all kinds of aquarium vampires.