It's camillanus, a type of worms. If you can get levamisole from your vet that is best, treat in the water (not food) at dose vet recommends, I think it's 0.1mg/litre. Piperazine works too in the water.
Drontal is one tab per 40L I believe, Dogmatix knows. I tried it on my rams but it didn't seem to work, may have been too far gone though.
Camillanus
Camillanus is easily recognized as a small thread-like worm protruding from the anus of the fish. Control of this nematode in non-food fish is with fenbendazole, a common antihelminthic. Fenbendazole can be mixed with fish food (using gelatin as a binder) at a rate of 0.25% for treatment. It should be fed for three days, and repeated in three weeks.
(http://www.simplydiscus.com/library/dis ... ro11.shtml)
Also, how to add fenbendazole aka panacur to food here: http://www.simplydiscus.com/library/dis ... worm.shtml