Fish Lice

When to act urgently

Breathing distress, rapid deterioration, inability to remain upright, major wounds or multiple deaths require urgent aquatic-veterinary or fish-health help. Check aeration and water immediately; do not wait to match a photograph.

What you may notice

• Flattened moving parasites, rubbing or attachment wounds may be seen.

Possible causes and look-alikes

• Argulus are crustacean parasites. Irritation without an identified attachment has many other possible causes.

Check first

• Record species, aquarium type, recent arrivals or changes, appetite, breathing, and how many fish are affected. Check aggression, injuries, filtration and aeration.
• Test ammonia, nitrite, pH and temperature; measure dissolved oxygen where possible and salinity in marine/brackish systems. Correct verified environmental problems using suitably prepared water, avoiding abrupt chemistry changes.
• Inspect affected fish and new-stock history; avoid sharing wet equipment between systems.

Treatment and correction context

• Obtain identification and a whole-system plan covering off-fish eggs and other life stages.
• Ulcers, weakness or breathing difficulty need prompt assessment. Removing a visible parasite does not by itself establish clearance.

Record for your next step

Write down onset and progression, species, affected/total fish, recent introductions, feeding observations, dated water results with kit units, and every action or product already used. Contact the diagnostic service before collecting or shipping samples.

Sources and scope

Source checks: 2026-10-03. Educational editorial summary; no independent veterinary review.

• UF/IFAS: Argulus Infections

  Fish-louse identification, eggs, lifecycle and management.

• UF/IFAS: Submission of Fish for Diagnostic Evaluation

  Case history and contacting a diagnostic service before collecting samples.