Levamisole Hydrochloride (Levamisole HCl), is an anthelmintic (anti-worm) agent commonly used in large livestock such as cattle, pigs and sheep. 
Currently, Levamisole HCl is used in humans for diseases related to imbalances in the regulation of immune responses or deficiencies of the  immune system, including autoimmune diseases, chronic and recurrent diseases, chronic infections and cancer.

It has beneficial effects on host defense mechanisms and restores depressed immune responses in animals and human.
Another interesting use of levamisole in humans is as a treatment for common warts (verruca vulgaris).
News of its anthelmintic efficacy and immune system benefits has been known among aquatic hobbyists for years.
As an added benefit, it boosts the immune competence of fish, humans, large animals, birds and some reptiles. This article hopes to pull together what scientific information is available, a bunch of individual accounts, apply some common sense, and put together in one place a helpful guide to the use of Levamisole HCl for treating aquarium fish.