http://www.simplydiscus.com/library/disease_medications/internal/nematodes6.shtml
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link goes for several pages, so this post has only partial info)
Nematode (Roundworm) Infections in Fish
Page 6 of 11 Pages: 1 . 2 . 3 . 4 . 5 . 6 . 7 . 8 . 9 . 10 . 11
Treatment
Prior to administration of deworming medications (anthelminthic), the problem should be properly diagnosed by a fish health specialist. After verification of infection, alternative therapies can be discussed. Food fish and game fish producers will have more limited treatment options than those available to ornamental fish producers. There are no Food and Drug Administration (FDA)-approved drugs for use in the treatment of nematode infections in baitfish, ornamental fish, or food fish, but ornamental fish are considered a low regulatory priority by the FDA.
For intestinal nematode infections of ornamental fish, several anthelminthics (dewormers) are available. Two effective and commonly used dewormers are fenbendazole and levamisole. Fenbendazole can only be used as a feed additive at the rate of 1.14 grams per pound of food fed for three days, with a repeat treatment in two to three weeks. Levamisole can be used both in the feed and as a bath treatment. One effective oral dose is 1.8 grams of levamisole per pound of food fed once a week for three weeks. One effective bath treatment dose is 2 ppm (mg/liter) levamisole for 24 hours, with a repeat treatment in two to three weeks. However, be sure to consult with a fish health specialist before commencing any treatment and always follow their recommendations.
Control of nematodes that infect areas of the fish other than the gastrointestinal tract is more problematic. For example, Eustrongylides species are typically found encysted within the body cavity outside of the intestinal tract, so common dewormers are ineffective. In fact, surgical removal is the only way to rid fish of internal worms that are not found in the intestine. Fish that contain nematode larvae in their muscles or external areas may be treated by surgical removal of the larvae. This approach is not practical aquaculture production, and sites of surgery may be unsightly or become infected. In general, affected fish must be culled.
Prevention
Prevention is always the best option, especially against those species of nematode that infect areas other than the gastrointestinal tracts of fish. Ponds that have not been cleaned or sterilized prior to restocking are at greater risk of harboring numbers of intermediate hosts (e.g., tubifex worms). Cleaning and sterilizing ponds is an effective way of reducing the numbers of the intermediate hosts of some nematode species. Preventive medicine can also help the producer avoid illness and production declines.
Preventive medicine includes periodic checks of susceptible juveniles or brood stock (by regular fecal examinations and necropsies of small numbers of fish) and reducing the presence of intermediate hosts, including fish-eating birds and tubifex worms, as mentioned above.
Producers who use live side swimmers (such as Gammarus spp.), insect larvae, tubifex worms, or copepods as food must also be aware that they may be, inadvertently, introducing nematode larvae into their stocks. So, even if these producers obtain their live food from a source that guarantees it is not grown in water containing fish, accessible by fish-eating birds or contains other hosts (all sources of possible infection), it is still important for these producers to practice preventative techniques, such as fecal checks, pond sterilization and necropsies on a regular basis.
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