Fish Lice Argulus
Whether you call it Fish Lice or Argulus it has the same symptoms and treatment.
This is very easily identified by circular parasites attached to your fish’s skin. Irritation and ulcers normally develop in the area of the parasite and are visible to the naked eye.
These parasites came with one of your fish when you purchased it. The problem is the adult parasite lays its eggs within the tank on the decorations and substrate. These eggs can stay in the tank for an extended period of time because they usually do not hatch until the temperature in the tank rises. Because of these factors fish louse are hard to prevent but also very hard to completely rid your self of when you they do show up.
The only effective way to treat your fish from louse is by physically removing the parasites from all stricken fish with a pair of tweezers. Any wounds that are caused by the parasites should be fully cleaned with a cotton swab saturated in Mercurochrome. You basically will have to start your aquarium all over by throwing out all decorations and substrate and removing all water.