10 Most Common Worms and Parasites in Freshwater Aquariums & How to Treat Them


Freshwater aquariums, unfortunately, are not immune to worms and parasites. Sometimes these pests accidentally end up in your aquarium affecting the health of your fish. When they do become ill, it is important to provide them with the best medical care and treatment. Here are the top ten most common worms and parasites that may infect your aquatic fish and how to treat some of the most problematic ones.


What types there are:

  • Detritus worms
  • Planaria
  • Camallanus worms
  • Trematode worms/flukes
  • Crustacean parasites
  • Annelids
  • Nematode worms
  • Fungal infections
  • Protozoan infections
  • Piscinoodinium (known as velvet, rust, or gold dust disease)


Worms can be introduced in many ways to your aquarium and as they are nocturnal, they probably make an appearance when you are sleeping during the night. They are generally introduced when purchasing new fish and plants that have not been quarantined before you put them in your aquarium.


Which ones should you be concerned about?

Anchor worms

  • They are introduced when new fish and plants are added that are already infected or have larvae hitch-hiking on them.
  • They will anchor deep into your fish’s body (usually the fins and tail) weakening their immune system and causing secondary bacterial infections.
  • Fish will form ulcers and redness where the worm enters causing the body to become inflamed. They will also become lethargic and develop breathing difficulties.
  • Potassium permanganate is the best method for removing them, but you can also physically take them off. Just be sure not to injure the fish and be sure to remove the entire worm.
  • You can also do a salt-water bath for the fish and use anti-parasite treatments.

Flukes/trematode worms

  • These worryingly can even infect a healthy fish. They hook onto your fish’s body and drill into the skin leaving lesions which can cause secondary infections. When they attach to the fish’s gills, they reduce the ability for your fish to take in oxygen so gills may be seen pulsing rapidly.
  • A layer of mucus will build up around the gills and cause the gills to become frayed. Small blood spots may also be seen where the worms have entered the body.
  • When they rapidly reproduce the cause of fish death is usually from a secondary bacterial infection or suffocation.
  • They make an appearance when your aquarium becomes overcrowded and has poor water quality, especially when ammonia levels become too high.
  • Firstly, you should eliminate any stress that could affect the fish. Then use an anti-worm treatment such as praziquantel. This treatment is great as it has no negative impact on fish, plants, or filters.

Camallanus worms

  • They occur when introducing plants and fish that have not been quarantined and fish food containing crustaceans as camallanus worms usually infect them.
  • This infection can be fatal if not treated quickly.
  • These are found inside the fish and can sometimes be seen hanging out the fish’s anus. The fish will become bloated around the abdomen and have a loss of appetite.
  • Antihelminthic medications must be used to treat this infection. This medicine is toxic to snails and shrimp, so remove them before treating the aquarium.


  • Leeches can suddenly appear in your aquarium when live food is fed to your fish, or when new fish or live plants are introduced. Due to their large size, they are easily spotted. They attach themselves to the fish’s skin and gills causing irritation. You must never manually remove leeches as they bury deep into the fish’s skin, if you try to remove them it can seriously damage the fish.
  • The fish will have small blood spots, be seen rubbing against objects inside the aquarium and have pulsing gills.
  • Bath the fish for 10-15 minutes in a 2.5% solution of saltwater as this should make the leeches release their grip (do not do this with salt-sensitive fish).
  • Medicate the whole aquarium with 0.25 mg of Trichlorfon per litre of aquarium water.

Fish lice

  • These are difficult to spot as they take the same colour as their host.
  • They usually attach behind the head where they will be feeding off your fish. This means the fish are prone to secondary parasitic and bacterial infections. These parasites burrow into the fish’s eyes and enter their muscles where the parasite lays its eggs where further infections can occur.
  • The fish’s skin will become pierced and inflamed with ulcers present. They will become stressed and aggravated followed by rubbing against the glass and any objects inside the aquarium.
  • You can manually remove them and clean around the wounds with an antiseptic treatment. Specialised medications such as dimilin can also be added in the aquatic water.

Ichthyophthirius multifiliis (ich)

  • Commonly known as white spot disease. This is one of the most common parasitic diseases and is difficult to control once the parasite is present in the aquarium due to a rapid reproductive rate. If not controlled quickly it can be transferred to other fish and will kill all your fish in the aquarium. Scaleless fish such as loaches and eels seem to be more susceptible to contracting this parasite.
  • Small raised white spots will form on the fish’s body and gills. Their fins may also become clamped. Due to difficulty breathing the fish can be seen gasping for air at the surface, staying close to the filter and breathing rapidly.
  • When there is a sudden temperature decline (usually from a heater malfunction or adding cold water into the aquarium) an outbreak can occur. Also, a poorly maintained aquarium may be to blame.
  • Medication such as formalin can be bought and added into the aquarium, but aeration is required as the formalin removes oxygen from the water. You can increase the water temperature but remove fish if they cannot tolerate high levels. Adding aquarium salt or unionised salt has also been effective.


  • Commonly known as velvet, rust, or gold dust disease, this is a common and very contagious disease that can kill every living thing in your aquarium. It attaches to the skin and gills of the fish feeding off nutrients inside the body and destroying the cells. Eventually, they drop off after they are done feeding splitting into many cells.
  • Yellow or brown mucus will form on the skin, and the skin may even start peeling off. Fish usually become lethargic, lose weight, and stop eating.
  • It occurs in poorly maintained aquariums and sudden temperature changes. An introduction can be from plants and fish if not quarantined before adding them into the aquarium.
  • Copper sulphate can be added to remove them, but it is lethal to shrimp and snails, so be sure to remove them before. Other chemicals such as malachite green and acriflavine can be added to the aquarium water.

Hole in the head disease

  • Also commonly known as head and lateral line erosion, this is thought to be caused by a protozoan called Hexamita. Poor water quality, vitamin deficiencies and use of activated carbon in a closed aquarium system are also thought to be attributes.
  • Any open lesions create an entry point that pathogens can enter and cause secondary infections. Usually, it starts in the intestines and then spreads around their body.
  • The fish usually become weak, thin, bloated, and lethargic. You may notice white, stringy faeces from their anus and trouble swimming due to balance.
  • Adding metronidazole, carrying out a 30-50% water change and checking the pH & water temperature should be the first steps. Improve the fish’s diet with high-quality flake food and add vitamin supplements.


How to reduce worms and parasites?

Firstly, you will want to identify the worms/parasites to figure out the best way to remove them and prevent them from infecting your aquarium. You will want to look at their movement, body shape, and where they are in the aquarium.

Always ensure you have the best water quality environment to reduce stress in fish.

When buying plants, it is best to buy potted ware or in-vitro plants as they are cultivated in very sterile conditions and are guaranteed to be free of pests.

When purchasing new fish and plants, set up a quarantine tank and observe for any pests that may have accidentally arrived home with you.

Thank you for reading, we hope this has helped. We look forward to seeing you again soon!