Fish lice or carp lice (Argulus species) are flattened, disc-shaped crustacean ectoparasites (3–12 mm) that attach to the skin, fins, and gills of fish. Despite their name, they are not insects but crustaceans from the class Branchiura, closely related to crabs, lobsters, and shrimp, not true lice. They anchor to the fish using suction cups and hooks, then pierce the skin with a sharp stylet to feed on blood and tissue fluids. During feeding, they inject toxic digestive enzymes that cause severe localized inflammation, cell destruction, hemorrhage, and ulcers. Heavy infestations lead to significant anemia, weight loss, excessive mucus production, scale loss, and fin corrosion. A single louse sting can be fatal to small or juvenile aquarium fish, while larger fish suffer chronic debilitation. Infested fish frequently scrape or "flash" themselves against rocks, substrate, or tank ornaments to scratch the intense irritation. Affected fish often show loss of appetite (inappetance), slowed growth, erratic swimming, and weakness. A person removes fish lice from the head of an aquarium fish. After being dislodged, those greenish lice move rapidly in the water in Tehatta, West Bengal, India, on September 6, 2026.
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