The ergasilus parasite is also commonly known as gill ice.This parasite is very similar to anchor worm but only attacks the gills of the fish.
The most noticeable symptom of gill lice is whitish green threads hanging from the gills of the infected fish in your aquarium hence the name gill lice. They are very noticeable and very easy to diagnose.
This parasitic infection when caught early is a very simple to treat and rarely fatal. The best treatment is to use potassium permanganate on the whole tank at a ration of 2 mg per liter. Some people say to treat the fish individually in it own bath because it can be messy and does effect the color of your aquariums water. The reason I suggest the whole tank is that it will get all the parasites and reduce the risk of re-infection.