How Do Fish Get Parasites?

Fancy goldfish with Ich parasite

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Parasitic infestations are common in fish. Fish may have parasites on them all the time, but with a healthy immune system, they are able to keep the problem in check. A stressed fish will cause a drop in immune function, which allows the parasite to flourish. How should you protect your fish against parasitic infestations and what should you do if you suspect one is running wild in your tank?

Parasites Found on Fish

Depending on if your fish lives in saltwater or freshwater, they will be susceptible to different parasites. Thankfully, none of the parasites will jump between saltwater and freshwater. Almost all of them require a fish to complete their life cycle, so empty aquariums and filters will not sustain parasites very long. There are commensal worms and even protozoa that live in filters that are often confused with fish parasites. Some parasites may require an invertebrate host to complete their life cycle, and the parasite won't reproduce without this diversity in your aquarium. Common parasites in freshwater pet fish include protozoa white spot disease (Ich), Trichodina, Costia and Chilodonella, as well as the flatworm skin and gill flukes. In marine fish, you should be aware of Cryptocaryon, Uronema, and Marine Velvet, in addition to others. 

Most parasites on pet fish are external. Unless your fish has come from the wild, it is less common for them to have any internal parasites. There are intestinal worms, such as Capillaria and Camallanus, and the intestinal Protozoa Spironucleus that can occur inside fish. These may cause a change in feces, and the fish may lose weight. Clear fecal casts, commonly referred to as “white stringy poop,” can be confused for a parasitic worm. 

Parasites can also affect the gill tissue, which can provide a parasite with lots of direct nutrition. Gills are critical to adequate oxygen levels for your fish and parasites on them can cause reduced respiration and severe health issues. Some parasites are specific to gills, but high numbers of parasites on the skin can lead to them migrating to the gills. 

How to Prevent Parasites on Your Fish

It is impossible to ensure your fish is 100% parasite free. You cannot sterilize a fish in order to remove all its parasites, so providing a healthy environment for your fish goes a long way. Fish stay healthy with clean water and a high-quality diet. 

The most common way a parasite infestation takes off is when a new fish is added into an established system. The stress from capture, transport, and introduction to a foreign environment causes any parasites on the new fish to quickly spread to the others in the aquarium. Fish move through pet stores very quickly, so they may not look sick at first. After a few days of incubating on a fish, the parasites can reach critical levels that make your fish ill. 

In order to help your fish stay healthy, you must quarantine them for four to six weeks in a separate system. This will ensure that any parasites riding on the new fish will not affect the entire system. In quarantine, a fish can recover from their trip and have a space of their own with no competition for food. After the quarantine period, if they have not gotten ill, they can be moved to the main aquarium with proper acclimation. If they get sick in their quarantine tank, you will only have to treat a few fish in a smaller system. 

How to Know if Your Fish Has a Parasite

If your fish has a clinical parasite infestation, it may exhibit a variety of signs. Physically, you may not be able to see the parasite. Macroscopic parasites—such as fish lice and anchor worms—are large enough to be seen with the naked eye, but can be confused for other skin issues. Other signs of a parasite infestation include missing scales, bruising or increased mucus coat, often seen as frosting, white patches, or bumps. Individual white spots on the fish are a common sign of some skin parasites.

You may also see behavioral signs of disease. This includes lethargy, decreased appetite, or increased respiratory effort/rate. The fish may hold their fins against their body and swim in a "shimmying" fashion. Rubbing their body on rocks, or flashing, is a fish behavior utilized to scratch themselves. Severe infestations can cause sudden death. Parasites can spread between fish very quickly, especially in overstocked aquariums and ponds, so your fish may be seeing a variety of symptoms.

Treatment for Parasites

There is no one treatment for all parasites. There are medications that treat against protozoa and medications for external worms that contain Praziquantel. It is important to have a correct diagnosis to determine if the parasite is a protozoa, worm, or crustacean in order to provide the proper treatment. Most of the time, you will also need to correct any stress issues, such as poor aeration or water quality, in addition to treating the parasite itself. With new fish additions, you may need to limit competition for food, provide additional hiding spaces and avoid squeezing too many fish into one tank. Don't forget to quarantine new fish in a separate quarantine tank before putting them into your aquarium.

A correct parasite diagnosis should be performed by a qualified veterinarian. Dumping various over-the-counter medications into your aquarium may do nothing for the parasite issue if the correct medication is not used. Never use antibiotics for a parasite infestation. You will only make matters worse by wiping out your biological filtration and not doing anything about the parasite. Consult a fish health expert when you suspect a parasite on your fish to get the correct diagnosis and treatment.

Article Sources
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  1. Floyd-Francis, Ruth, et. al. Monogenean Parasites of Fish. University of Florida School of Forest, Fisheries, and Aquatic Sciences--Fisheries and Aquatic Sciences. April 28, 2019.

  2. Quarantining fish to reduce risk of illness. DVM 360. January 18, 2022.