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external parasite

Fish lice (Argulus)

Flat, disc-shaped crustacean parasites (1–10 mm) visible on the body or fins. Fish show intense irritation, flashing, and scratching. Argulus use sucking mouthparts to feed on blood and inject a toxin that causes lesions and immune suppression. Most common in ponds but can infect aquarium fish.

High priorityhigh confidence if visible1 source note

Do first

  • Examine the fish under bright light Argulus are visible to the naked eye as flat, rounded, semi-transparent discs.
  • Individual lice can be manually removed with fine tweezers while the fish is briefly, carefully restrained.
  • Begin antiparasitic treatment immediately to kill free-swimming larval stages in the water.
  • Improve water quality to help prevent secondary infection at bite sites.

Escalate if

  • Heavy infestation causing multiple large open wounds.
  • Fish in shock or showing severe lethargy Argulus toxin injection can cause systemic effects.
  • Secondary bacterial septicemia developing at bite sites.

Water clues

These readings can push this pattern higher or lower in the triage result.

ammonia above zero+2

Argulus bite wounds become secondary bacterial infection sites in poor water quality.

Care protocol

Follow only the steps that fit your species, tank inhabitants, and medication label.

Manual removal

  1. Argulus can detach and re-attach manual removal reduces immediate load but must be combined with chemical treatment.
  2. Grasp the louse firmly and quickly with fine tweezers and lift away in one motion.
  3. Examine bite sites afterward apply topical antiseptic to any open wounds.
  4. Recheck daily for newly attached individuals.
Cautions
  • Argulus move quickly work fast and minimize fish stress during handling.
  • Manual removal alone will not clear an infestation eggs and larvae in the water will reinfest.

Chemical treatment

  1. Diflubenzuron is effective against larval stages treat the tank according to product instructions.
  2. Organophosphate treatments (e.g., those containing trichlorfon) can be effective but require precise dosing.
  3. Treat in multiple rounds 1–2 weeks apart to address all life stages.
  4. Vacuum substrate and perform water changes between treatments.
  5. Treat secondary bacterial infections at bite sites with a broad-spectrum antibacterial.
Cautions
  • Organophosphates are highly toxic to invertebrates remove all snails and shrimp before treatment.
  • Do not dose above instructions overdose is fatal.
  • Check plant compatibility before treating.

Source notes

References and context notes used for this triage entry.

Argulus (Fish Lice) UF/IFASUniversity of Florida EDIS extension reference for external crustacean parasites.
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Related patterns

Similar symptoms or risk factors worth comparing before treatment.

Medium priorityAnchor worms (Lernaea)

Visible worm-like parasites embedded in the flesh, often with inflamed red attachment sites. Lernaea are actually crustaceans, not true worms. Most common in goldfish, koi, and other pond fish but can affect aquarium species. Highly visible to the naked eye once adult females are attached.

UrgentUronema marinum (marine ciliate)

A scavenger ciliate that becomes pathogenic under poor water quality or stress, causing rapidly spreading open lesions, internal hemorrhaging, and fast death in marine fish. Fish appear to melt. Uronema is most dangerous to thin-bodied fish (chromis, small wrasses, clownfish) and is often introduced with uncleaned live rock or wild-caught fish. Notoriously difficult to treat.

UrgentMarine velvet (Amyloodinium ocellatum)

The marine equivalent of freshwater velvet an extremely fast-moving dinoflagellate parasite causing gold or rust-colored dust on marine fish, rapid breathing, and quick death. Amyloodinium is considered one of the most dangerous diseases in the marine aquarium hobby and can kill an entire reef tank's fish population within 2–3 days if untreated.

High priorityGill flukes or body flukes (Dactylogyrus / Gyrodactylus)

Rapid breathing, gill flaring, flashing, and scratching caused by microscopic monogenean parasites on the gills (Dactylogyrus) or skin/fins (Gyrodactylus). Often introduced with new fish. Requires microscopic confirmation for certainty but may be treated empirically when symptoms are consistent.