Fish lice are known as Argulus. They are parasitic crustaceans that enter your fish habitat hitching a ride on new fish or plants.
They are usually found just beneath the fish's scales, behind fins or near eyes and gills. The lice can appear nearly transparent. They feed off the fish by injecting digestive enzymes into the skin of the fish which liquefies the flesh. A straw-like appendage is able to suck out the liquefied flesh.
Fish with lice become more susceptible to bacterial infections, skin ulcers, may develop anemia and experience tissue damage and scarring.
If your fish has tiny, red, irritated-looking spots or they try rubbing their bodies against something, you can suspect lice.
One of the older methods to remove the lice is holding the fish in a damp cloth and removing the lice with tweezers. There are now medications that treat the fish and the entire tank.
To prevent lice from taking hold, carefully examine any new fish or plants before placing them in your tank or pond.
Fortunately, while there has been an increase in the incidence of fish lice, it still doesn't occur very often.