Nematodes In Aquarium – What Are They & How To Get Rid of Them

Have you noticed any white tiny worms in your aquarium? You might need a strong magnifying glass to see them, but symptoms in your fish’s behavior and appearance could also help you determine whether or not these are nematodes that have infested your tank.

Although introducing fish into the aquarium can help reduce the number of nematodes, this can be dangerous to these aquatic creatures because when ingested, nematodes can end up eating essential organs or infecting your fish’s gut which could be fatal. These parasites can eat them from the inside out.

So if you’ve noticed nematodes in your aquarium, this guide will help you understand exactly what nematodes are and how to get rid of them without harming your fish any further.

What are Nematodes? 

Nematodes are small cylindrical parasitic worms found almost anywhere in the world, especially in soil or water. Some of these worms are commonly known as roundworms and can harm fish, plants, and other domesticated animals.

Types of Nematodes

There are two common types of parasitic Nematodes you’re likely to find in your aquarium. These are the Camallanus and Capillaria nematode worms. 

Camallanus Worms

Camallanus worms are cylindrical and similar to tapeworms in appearance. They have shorter and segmented bodies and are likely to be found in fish tanks with low oxygen levels. You will mostly find Camallanus worms in larvae form that eventually find their way inside the fish’s body through the water they ingest.

Camallanus Worms

These worms will appear reddish brown in color. They are either found in your fish’s feces or extending from their anus. You need to act quickly if you see multiple Camallanus worms or notice swelling in your fish’s anus.

The ones protruding from your fish’s anus are most likely female worms thriving in your fish’s intestines and guts but are now releasing larvae into the water. When released into the water, they can easily be ingested by the fish, and the cycle continues.

Capillaria Worms

Capillaria worms are different from Camallanus worms. Although they both exhibit similar symptoms when they infect the fish, the difference is Capillaria worms do not protrude from your fish’s anus to release living larvae. They will lay eggs that you will likely find in your fish feces.

They are primarily found in fish tanks with low oxygen levels and cloudy water without filters. Unlike Camallanus worms, which primarily attack freshwater fish, Capillaria worms infect all fish.

Symptoms like bloating, white feces, and lack of appetite indicate a possible Capillaria worm infestation in your tank. Most aquarists notice something is wrong when many fish die one after another. Some aquarists have found these worm’s eggs in the bodies of their dead fish.

How Do Nematodes Get Into Your Aquarium? 

There are many ways in which nematodes can get into your aquarium. It can be through infected fish, the substrates, or plants introduced into the tank.

Via the fish

You might introduce a new fish to your community tank that has been infected but isn’t exhibiting any symptoms. Unless you have a magnifying glass, it isn’t easy to detect nematodes in your fish until they have begun affecting the aquatic life in your tank.

They are tiny and could be in their larvae stage, making them even harder to identify. Quarantining a new fish is the best way to prevent a nematode worm population in your fish tank.

Via the plants

It’s common for worms to get into your tank by adding new plants. The worms may secure themselves under the plant leaves or roots and get into the substrate once you settle the plant in the fish tank.

Via the substrate

A dirty substrate can also be how nematode worms make it into your fish tank. If you don’t clean the substrate before adding it to your fish tank, you could end up adding these parasitic worms into your aquarium.

If your aquarium has low oxygen levels, dirty water filled with food waste, and high ammonia levels, nematodes will likely thrive and reproduce faster. A population explosion will only lead to more infections. 

Are Nematodes Harmful To Fish? 

Yes. Aquarium fish infected by nematodes are likelier to die because these parasitic worms infect the fish’s intestines and guts. Nematodes are primarily found in dirty aquarium water, so if you haven’t been cleaning your tank, it’s likely that your fish’s immune system has also been affected and now they are more susceptible to infection from nematodes.

It’s more likely for nematodes to infect freshwater fish like cichlids, so you should keep a closer eye on them and ensure to maintain the proper water parameters in their tank. While isolated infections aren’t fatal, poor water conditions can lead to a growth in their population and numerous infections in other freshwater fish.

An infected aquarium fish has lower immunity making it easier for the worms to damage their internal organs.

Signs of Nematode Infections in Fish

If you suspect that your fish have been infected by the nematodes, here are signs you should look out for:          

  • Camallanus worms (reddish brown worms) protrude from the fish’s anus. The anus could also be irritated and swollen.
  • Loss of appetite
  • Capillaria worms are found in a dead fish’s body.
  • Bloating, caused by nematodes in the fish’s gut and intestine.
  • White feces
  • Lethargy 
  • Odd swimming patterns
  • Sudden death
  • Sinking stomach
  • Rubbing against objects in the tank. 

All of these could also be signs of discomfort or other illnesses in your fish, so you need to find the nematodes to be sure that this is the cause of their change in behavior and other symptoms.

The nematodes may have been in your fish tank for a long time, but you didn’t notice them as they can sometimes be pretty hard to spot.

Nematodes can survive in a well-maintained tank, but they will probably not thrive to the extent of infecting your fish and affecting their health. 

How to Get Rid of Nematodes in Fish Aquarium

If you’ve done your research and are sure there is a nematode population in your tank, you need to start planning how you’ll get rid of them.

If you have numerous fish in your tank, the first step should be to move the infected fish to a quarantine tank, but this may not be enough because you’ll only notice symptoms long after infection and by this time, it could already have infected other tank mates.

Using Medication

The best way to eliminate nematode worms is by disrupting their life cycle. This will prevent the further spreading of the infections and prevent them from reproducing. You can do this using medication that effectively kills nematode worm infection, like Fenbendazole and piperazine. These medications are effective when ingested and not added directly to the water.

Regular Aquarium Maintenance and Cleaning

The surest way to get rid of nematodes is to keep the tank water clean. Constant cleaning, frequent water changes, and a filter can help reduce waste in the tank and boost your fish’s immune system.

If you’re using a heater which is likely the case in a freshwater tank, you need to keep the water at optimum levels and not too warm because nematodes thrive in warmer temperatures. 

Good Quality Filtration System

A reliable filter will help your fish breathe better and increase oxygen levels through proper aeration. A good filtration system will also remove excess dirt from your fish tank. Without uneaten food and waste, the nematodes will likely starve, which limits their spawning.

Substrate Vacuuming

Whether or not you have parasitic nematodes in your fish tank, you should make it a habit to vacuum your substrate.

Substrates are a breeding site for most worms, and the buildup of waste can increase ammonia levels. This will help remove uneaten fish food, prevent algae overgrowth, and keep the tank clean.

Treating Nematodes in Fish Aquarium 

One of the most common ways to treat nematode infection in your aquarium is by using chemicals. Some of the most effective chemical treatments include Fenbendazole, Pyrantel Pamoate, and Methylene Blue. 

Most aquarists recommend taking the advice of a veterinarian before purchasing or administering any of this medication. The medication doesn’t kill these parasites; it only paralyzes their jaw to prevent them from causing more harm to your fish’s internal organs and body. 

The worms are then pushed out of the fish’s body so you can vacuum them out of the tank. 

An effective medication you can use to treat an aquarium is Levamisole. This medication is readily available, but you should be careful not to order the wrong type. 

To treat the fish tank, add 2mm of the medication, i.e., Levamisole, for every liter of water. For example, if you have a 25-liter tank, you’ll have to add 50 mm of the medicine to the water. After a day, you can carry out a 60% water change, then vacuum the gravel and substrate to ensure they’re all gone. 

Getting rid of all the nematodes in your tank can take weeks to months, so you shouldn’t lose hope too fast. For some fish, once the infection is severe and has affected significant organs, you can’t do anything more to help. 

Giving Nematode Medication to Fish

The Fenbendazole Medication for these fish should be administered with their food to help eliminate the infection. Don’t, under any circumstances, add the medication directly to the water. With most of this medication, the molecules are too large and unable to be absorbed through the fish’s skin or their gills from the tank water. So the best option is through their food. 

It’s not difficult to make medicated food for your fish; you have to mix the food and the medicine in a ratio of 1 to 5. Mix the medication in hot water and thoroughly stir it. Then add two tablespoons of fish food until you get a paste-like mixture. You can add more food if the mixture is too watery to achieve a thick consistency. 

Getting your fish to eat the medicated food won’t always be easy, but you can leave it in the tank so that they eat it whenever they’re ready. If you’re giving the medication to a large number of fish over a long period, it’s best to keep the food in a plastic bag in the freezer.

How to Prevent Nematodes in Aquarium

Once you have treated the nematode infection in your fish tank, You need to ensure that it doesn’t happen again. You can only do this through intensive aquarium cleaning and maintenance.

Here are some best ways to prevent another nematode infection in your fish aquarium.

  • Clean the tank and remove any uneaten fish food because this leads to an increase in ammonia levels and a decrease in oxygen levels, which creates the perfect environment for nematodes to thrive.
  • To avoid a buildup of uneaten food in the tank, ensure you only give your fish the amount of food they need. Overfeeding leads to an increase in debris and waste in the fish tank.
  • Clean the substrate and gravel because most nematodes are generally found hiding there. 
  • Change and clean the filters in your fish tank. Most parasites, including nematodes, hide here because of the many bacteria.
  • Ensure you maintain the proper water parameters, especially in a freshwater tank, because nematodes thrive in dirty water filled with food waste.
  • Before adding a plant ornament or substrate to your aquarium, ensure that you thoroughly clean them because this is one of the ways that nematodes make their way into your aquarium. Or you can use hydrogen peroxide to deep decorations and live plants as long as you thoroughly clean them before placing them in the main tank.


Are nematodes harmful to humans? 

Yes. Certain nematode species can infect humans by ingesting raw fish. Symptoms in humans can include nausea, weight loss, vomiting, abdominal pain, and diarrhea.

What are detritus worms?

Detritus worms are thin parasitic worms that feed on waste and decomposing plant matter in the fish tank. However, they will not harm your fish.

How long can Camallanus live without a host? 

Without a host, the Camallanus worm can only survive for a week. Although they don’t live long, the female Capillaria worm produces over 300 larvae that can live for months before getting into the fish’s body.


Freshwater fish are very vulnerable to nematode infections, so you should be extra careful to prevent the death of your adorable fish. Although Capillaria and Camallanus worms are common in aquariums, you are more likely to find the Capillaria nematode in your fish tank.

If you maintain a clean tank, you don’t have to worry about the nematodes in your aquarium, but dirty water can increase the population and lead to widespread infection.

Using the tips listed above, you can eliminate nematodes using chemical medication and prevent nematode infections in your fish. If you keep your tank water clean and monitor what you add to your tank, you won’t have to worry about a nematode infection in your fish tank. 

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