Anchor worms are large copepod parasites that embed their head into a fish's flesh, leaving a worm-like tail visible. Most common in pond fish (goldfish, koi) and outdoor-collected feeders. Adult anchor worms reach 1/2 inch and cause visible irritation + secondary bacterial infections.
Manual removal: net the fish, use tweezers to grasp the anchor worm AT the attachment site (not the tail), pull steadily. Dab the wound with iodine or 3% hydrogen peroxide. Then dose the entire tank with API General Cure or Dimilin (diflubenzuron) for 7 days to kill larval stages. Repeat in 14 days.
Anchor worms are rare in saltwater. When present, same protocol.
Quarantine all new pond fish 4-6 weeks. Avoid wild-collected feeder goldfish. Inspect new arrivals carefully under bright light. UV sterilizer kills free-swimming larval stages.
Estimated cost: $20-40.
Adult Lernaea reach 1/4 to 1/2 inch (5-12mm) and are clearly visible against the fish's body. Larval stages are microscopic and free-swimming - that's why tank treatment is required after manual removal.
The single most important disease-prevention step: a 4-6 week quarantine of every new fish before adding to your display. See the complete quarantine protocol.