Fish lice, otherwise known as Argulus, are disc-shaped crustaceans that feed on fish by injecting a digestive enzyme into the skin or gills of a fish and then sucking out the putrefied flesh. They are visible to the naked eye, usually measuring 1/5 of an inch to 2/5 of an inch (5mm to 10mm), and are a common parasite in ponds and fish tanks. Fish lice feeding on fish can also open the door for secondary infections.
Biologically, fish lice are classed as Branchiura, or fish parasites. They are also biologically classifiable as Crustacea, along with creatures such as shrimp, prawn and water fleas. Characteristics of crustacean creatures include a rigid chitin exoskeleton, which must be shed as the creature grows larger.
Fish lice mate when on the host fish, and afterward the female goes to lay eggs on plants or other objects toward the bottom of the water. The eggs are generally laid in the winter and hatch around the spring, when water temperatures rise. The creatures go through several metamorphic changes in their development, and usually secure a fish-host within four days. They continue to grow while on the fish, and the growth cycle can take anywhere between 30 and 100 days, depending on variables such as temperature.
There are two large suckers on the body of fish lice used for feeding in conjunction with their slightly smaller mouth. This causes the fish host both irritation and stress, and the affected area usually becomes inflamed. The areas of tissue damaged by the fish lice also give bacteria a chance to infect the fish, possibly leading to conditions such as skin ulcers and gill disease. The stress caused by fish lice can lead to secondary parasitic infections such as Costia and white-spot. It is also a possibility that infections are injected into the fish as the lice feed.
The most common treatment for fish lice is organophosphates, but many of these are banned in the U.S. for home and garden use. Three treatments of organophosphates over the life cycle of fish lice is generally enough to wipe them out entirely. Using a chitin inhibitor is an alternative solution, which prevents the young fish lice from developing when they malt their exoskeleton; this treatment is not toxic to fish. Chitin inhibitors can remove all traces of crustacean parasites in as little as five days.