Lice in Freshwater Fish

Fish lice, properly called Argulus, are unusual among aquarium parasites in being large enough to see clearly with the naked eye, flat, roughly circular, and often visibly moving across a fish's body or the aquarium glass. Despite the name, they are not insects but crustaceans, and they are considerably tougher to eliminate than their size might suggest.

Recognising an infestation

• Flat, disc-shaped parasites, typically translucent to pale green, a few millimetres across, visible on the skin, fins or gills
• Vigorous rubbing and scratching against tank decor and substrate
• Erratic, agitated swimming
• Small red or irritated spots at attachment sites, visible once the lice themselves are found and examined

How lice actually feed, and why it matters

Argulus has a piercing stylet it uses to puncture the fish's skin and feed on blood and tissue fluid, which causes significant irritation on its own and also creates an open entry point for secondary bacterial or fungal infection. In heavy infestations, lice can attack the gills directly, a genuinely dangerous progression that can prove fatal if left untreated.

Why lice are more stubborn than most external parasites

A female fish louse lays her eggs on hard surfaces around the tank, rocks, decor, glass, rather than on the fish itself, which means treating only the water or only the fish misses a significant part of the life cycle. This is also why lice can persist in a tank even after visibly affected fish appear to recover, since eggs continue hatching and reinfecting from the environment.

Treatment

• A salt bath at roughly two to three percent salinity for 10 to 15 minutes can help dislodge lice from an individual fish, used in a separate container rather than the main tank
• Purpose-made lice treatments containing diflubenzuron or similar compounds that disrupt the parasite's life cycle are the most reliable chemical option and are available through specialist Indian aquarium and pond supply shops
• Since eggs are laid on hard surfaces rather than the fish, a genuine infestation needs equipment and decor addressed as well as the water: cleaning or replacing substrate and thoroughly cleaning decorations reduces the reservoir of eggs waiting to hatch
• Repeat treatment is often necessary, since most products affect only certain life stages, and a single dose can leave eggs or juveniles unaffected to restart the problem

A note on where lice tend to come from

Argulus is considerably more common in outdoor ponds than in indoor aquariums, and the most frequent way it reaches an indoor tank is through introducing pond-raised or wild-caught fish, sometimes labelled simply as feeders, without proper quarantine. This makes quarantine particularly important for anyone adding pond fish or feeder stock to an established indoor aquarium.

Prevention

Quarantine every new fish for two to four weeks, inspecting closely for any visible parasites during that window, and avoid introducing untreated water or wild-caught live food from outdoor sources. Given how much harder lice are to fully clear once established compared with catching them during quarantine, this upfront caution is worth far more than the time it costs.

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