Not all nematodes are a menace, scavenger nematodes can help break down the organic waste in the aquarium. Some species are parasitic worms that can inhabit the intestines of tropical fish, often appearing in the anus of the fish. If you are treating your fish tank for nematodes, make sure that they are the parasitic kind. Anti-worm medication developed for aquariums are usually affective against nematode worms.
Cause: Nematode worms can easily be introduced to an aquarium when adding new fish or if feeding with live or raw food. Nematodes can also occur when the fish are being over fed and there is a large waste build up in the gravel.
Treatment for scavenger nematodes: Although scavenger nematodes are harmless to fish, they can be rather unsightly in your aquarium. Reduce feedings, increase water changes and gravel vacuuming. Add a species of fish such as Gourami or Pleco which will eat them.
Treatment for parasitic nematodes: If the worms appear to be coming from the anus area of fish then more drastic deworming treatment will be required.
Try dosing the feed in Fenbendazole and feeding this to the fish for 3 days. An option is to dissolve 3CC of powdered Fenbendazole in 100ml of water. Mix well, then add as many bloodworms as you need to feed your fish. Soak for 30 mins to 1 hour, then dump the entire container in the tank, water and all. It should be fed for three days, and repeated after three weeks.