Argulus, or fish lice, can cause morbidity and mortality in many different fish species. Fish lice are crustacean parasites that are related to prawns, shrimp, and water fleas. They have a direct life cycle which means that they use only the fish as a host. Argulus are visible with the naked eye and are approximately 5mm to 10mm in length. When they are attached to a fish they may simple look like black spots, until you notice them moving. The lice attach themselves using curved hooks and suckers and are usually located behind the fins and head. The best way to get rid of the lice is to treat the water using an organophosphate. This should be applied to the water three times, ten days apart.