Leeches

Signs

• Segmented worms with a sucker at each end, up to 5 cm, visible to the eye on skin, gills or inside the mouth
• Anemia and weight loss from chronic blood loss
• Heavily infested fish may swim at the surface with the back exposed and get ulcers on exposed skin

Cause

Blood-feeding parasitic leeches (Hirudinea). Infestations are most common in wild fish; aquarium and pond infestations come from introducing infested fish and plants.

What to do

Merck: organophosphate baths (0.25 mg/L, prolonged) are only 'somewhat effective', are not approved for food fish and may be restricted outdoors; multiple treatments may be needed because leech eggs are resilient and juveniles keep hatching.

If a fish is badly ill, talk to an aquatic vet.

Prevention

Avoid introducing leeches: effective quarantine of new fish and plants.