Fish lice are Argulus, a crustacean. These are rarely seen if tropical fish, but they can occur, especially in pond raised fish. Since true lice are insects the term “fish lice” is a misnomer.
There are several varieties of fish louse, including a very transparent one which is hard to see on the fish. Fish lice are sometimes seen on goldfish which have been raised in a pond. But fish lice as a whole are rare in an aquarium and will generally simply die out on their own.
Some hobbyists just remove the lice with the tweezers.
An as yet Undescribed Fish Louse
Another as yet described crustacean copepod is infecting just the fins of some fish. It originally was found on guppies from southeast Asia in 2002 (“The New Illustrated Guide to Fish Diseases”. Bassleer, 2008, p. 187). It has now been confirmed on goldfish, tetras and gouramis and appears to have an affinity for the fins of angelfish. It looks like a small one-millimeter long grain of rice or white barley. These small crustaceans appear to be relatively benign and are just confusing when they are encountered.
These white “pearls” are obviously a single type of organism, because of its very regular and equal sized shapes. Some have described it as a virus, lymphocystis, while others have described it as calcium deposits. Both these would appear as very irregular and different sized lumps and spots, so these observations are incorrect.
Treatment
Fish lice are arthropods and have an exoskeleton. In arthropods, exoskeleton development is interfered with by organophosphate insecticides. Fish lice are best treated with trichlorofon (Blue Planet Para Cide) and diflubenzuron (Dimilin-X). Put the insecticide into the water column. It won’t affect the beneficial bacteria in the filter. The treatment should be once a week for two months. Note Fritz Aquatics Mardel Clout (Metronidazole and Trichlorfon) is no longer being sold.
In some countries one is limited to garden insecticides such as Malathion. These can be added at one drop per ten gallons. Copper supposedly can also kill argulus at the dose recommended for ich. Note I have not been able to kill fish lice with copper, so I don’t know about this recommendation.
If the larval argulus have created ulcers or red spots it is best to treat with a broad spectrum antibiotic in the food as secondary bacterial infections can kill fish quite rapidly.
Bacterial diseases need to be hit with a broad-spectrum antibiotic. These need to be obtained from the internet. They include Thomas Labs Fish Mox, Midland Vet Service Aqua-Mox, VetDepot Amoxicillin, SeaChem KanaPlex, Fishbiotic Ampicillin, Mardel Maracyn 2, Thomas Labs Fish Min and Thomas Labs Fish Doxy. They are ONLY effective when mixed with the food (contrary to manufacturers profit driven instructions).
It is easy to make medicated food. Heat 1/4 cup water (two ounces or 58 milliliters, not a lot) in the microwave. Then blend one 1/4 ounce of plain animal derived gelatin (Knox gelatin, one envelope) into the hot water with vigorous stirring. Take two tablespoons of dry commercial fish food (pellets or flake) and mix it with just a little of the hot water/ gelatin mixture. Add hot water/gelatin until you get a paste like consistency. If it gets too watery just add more food.
Then add roughly 1/16 teaspoon (a 1% to 2% addition) of the medication to the mud. Mix and mash the whole mass thoroughly. Spread it out into a pancake about 1/8th inch (3 mm) thick on a plastic film or a plate. Then put in the refrigerator. If you plan on keeping it for more than two weeks put it in a small plastic bag and freeze.
Feeding should be at least for a week and longer if the fish doesn’t respond.
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