Large external parasites, such as leeches and fish lice, most commonly affect pond fish. They can cause a variety of problems, including weakening of the host fish, secondary infections in the fish, such as fin rot, and the transmission of blood-borne diseases between fish.
Not all leeches are parasitic, but many are. Parasitic leeches feed on the blood of the fish, leaving behind distinctive circular lesions on the fish’s body. These lesions appear sore and inflamed, and are frequently sites of secondary infection.
Anchor worms (Lernea spp.) are thin, wormlike crustaceans less than an inch in length. They are often black in color, and when they are mature, a pair of large egg sacs will be clearly visible. As with leech bites, the sites of anchor worm attachment in fish are bloody and frequently become infected.
Fish lice (Argulus spp.) are flattened, disk-shaped crustaceans less than half an inch in diameter. They irritate the host fish, causing it to scratch itself against rocks or repeatedly jump out of the water in an effort to dislodge the parasites.
Gill maggots (Ergasilus spp.) are closely related to fish lice, but as the name suggests, they infect the gills, rather than the body of the fish. When the fish’s gill covers are raised, the paired white egg sacs on the parasites will be visible as maggotlike structures.
All these parasites feed on the fish’s blood, weakening the host fish to some degree. In extreme cases, the host fish becomes emaciated and prone to secondary infections and other diseases.
Anchor worms, fish lice and gill maggots spend their entire adult life attached to a single host fish, but leeches will feed on a succession of hosts and are believed to be able to transmit blood-borne diseases between fish.
Leeches alternate between feeding and resting stages throughout their lives.
Anchor worms, fish lice and gill maggots have life cycles that involve a free-living juvenile stage and a parasitic adult stage. The precise details vary from species to species; among gill maggots, for example, only the female is parasitic.
Anchor worms and fish lice can be removed from the fish using forceps, but after removal, an antiseptic such as iodine should be dabbed on the wound to prevent secondary infections. Leeches can be removed in a similar way, though usually the leeches are weakened first by exposing them to a saltwater bath (see below). Gill maggots cannot be removed easily because of their location on the fish.
To remove leeches, place the fish on a soft, wet towel. Hold the fish firmly, but without squeezing. Only remove the fish from the water for a minute or two. Use the forceps to grip the external parasite as close to the body of the fish as possible, and then carefully pull it away. Dab the wound with antiseptic and then repeat the process for any other external parasites on the fish.
Physical removal of the parasites does nothing about those living free in the pond. To kill these, you will need to use an organophosphate insecticide.
Saltwater dips weaken or kill parasites, so they can be removed without damaging the host fish. In a bucket, mix 2½ to 4 ounces of aquarium salt per gallon of bucket-water at the same pH and temperature as the pond. Stir the water until the salt has completely dissolved.
External parasites on fish the size of koi or goldfish can be safely bathed in this saltwater solution for 15 to 30 minutes. Freshwater fish are stressed by immersion in saltwater baths, and if they show signs of severe distress, such as loss of balance, they should be removed at once.
The organophosphate insecticide metriphonate (also known as trichlorfon) is widely used to kill leeches and crustacean parasites in ponds and aquaria. It is not normally sold in pet stores but can be obtained through your vet. The dose depends on ambient conditions, particularly pH and temperature, but is typically around 1 mg per gallon for seven days. Activated carbon will remove insecticides from the water, so this will need to be taken out of any filtration systems if used.
Proprietary medications for treating some external parasites in fish are available in pet stores, in particular ones based on diflubenzuron (also known as dimilin). These should be used as instructed by the manufacturer.
Insecticides kill a wide variety of invertebrate life, not just parasites. Some are highly toxic to humans, as well as other pets.
Leeches and crustacean parasites mainly get into ponds and aquaria in three ways: attached to fish that haven’t been quarantined, in their free-living stages in packages of live food and attached to plants (especially in the case of leeches).
Always quarantine new fish before introducing them to your pond, and avoid using live foods. Never put aquatic plants collected from the wild in your pond, and instead use plants produced expressly for the fishkeeping hobby.