I've had to do it to save the others put a fish down with internal parasites, then stripped the tank down.
Camallanus are red worms that can be seen protruding from the anus of the fish, especially after eating and when the fish is at rest. From what I’ve seen, livebearers and Gourami seem to be the most susceptible to it. It is very difficult to treat and multiplies using an invertebrate as a host. Destroy infected fish and medicate to (hopefully) kill hosts and larvae. (or you can tear down your tank, sterilize your equipment, and start over). If you leave exposed fish in the tank, wait a few months before adding new fish.