Intestinal parasites are worms and protozoa (one-celled organisms) that take up residence in an animal's intestines. Once there, they get free room and board, sucking blood and body fluids from your precious pet. Here's the lowdown on the four intestinal parasites that commonly inhabit fish: tapeworms, flukes, roundworms and coccidia.


When we think of tapeworms, also known as cestodes, most of us imagine long, flat, wriggling worms that inhabit the intestines of dogs, cats or even people. But tapeworms go through their first stages of development in other species, such as fish.

Fish become infected when they eat a small crustacean, such as a water flea, that has dined on tapeworm eggs. The larvae inhabit the fish's muscle tissue, enclose themselves in a pouch and continue developing. In the wild, a bird, reptile or mammal eventually eats the infected fish, and the tapeworm larvae travel to the animal's intestine where they grow to maturity and reproduce.

How can you tell if your fish have tapeworms? You may notice tumor-like bumps on their skin. These bumps are developing larvae. Though the bumps are unsightly, the larvae generally don't harm the fish. If you really want to get rid of them, your veterinarian can prescribe a parasiticide called praziquantel. Giving your fish a one-time bath in this solution should kill the larvae.


Like tapeworms, flukes, a.k.a. trematodes, often use fish as intermediate hosts. Fluke larvae also enter a fish's muscle tissue or gills to continue their development.

You may see silver dollar fish with small black spots peppering their skin. These intriguing color mutations indicate infection with a fluke genus known as neascus. Another fluke genus, diplostomum, often invades a fish's eye causing blindness. Never purchase fish with white eyes, which are a sure sign of disease.

Several flukes in the heterophyes genus are particularly perturbing to fish fanciers. These parasites destroy the gills and leave fish clinging to life. Affected fish rarely withstand the stress of shipping, so it's unlikely you'll purchase one. But because fish acquire these flukes by eating snails, you should thoroughly rinse and quarantine any live plants you purchase for your home aquarium to prevent fluke-infested mollusks from sneaking into your fish's environment. As with tapeworms, a praziquantel bath can kill flukes.


Roundworms, or nematodes, inhabit various organs in fish. Some reside in a fish's stomach or intestines, where they scarf down the food your little swimmer ingests. In this case, your fish may look emaciated. Other roundworms harmlessly hibernate in muscle tissue or swim around in the abdominal cavity. If roundworms are holed up in the abdomen, your fish's belly may look swollen.

If you suspect most or all of your fish are infected with roundworms, you can treat the tank by mixing the dewormer called fenbendazole into their food. But be sure to stop feeding the fish three days before treatment to make sure they're famished since the medicated food isn't appealing.

Many owners of live-bearers, fish that bear live young instead of eggs, report seeing a wiggly red worm emerging from a fish's cloaca - the opening for the digestive, urinary and reproductive tracts. This is also a type of roundworm, but infections with these parasites usually clear up on their own.


Coccidiosis, an infection with the protozoa coccidia, occurs in goldfish worldwide, especially in early spring in rearing ponds where cool water may make the fish more susceptible to illness. If your fish have coccidiosis, their eyes may look sunken because of weight loss and you may see diarrhea in the form of fluid yellow feces.

Your veterinarian can identify coccidia eggs with a fecal exam. You can try to kill the parasite by mixing anticoccidial medications with food, but this treatment may not be completely effective.

Fish who survive the infection may still harbor some coccidia, which will shed their eggs in times of stress. Goldfish fanciers should monitor their fish's feces and treat the fish promptly at the first sign of disease.