Fish worms (internal parasites): signs, dewormers and safe treatment

Internal worms are a common reason a fish eats well but stays thin, passes white stringy poop, or develops a swollen belly. Most can be treated with the right dewormer, but you need to match the medicine to the type of worm, because no single product kills them all.

Many wild-caught fish, including plecos and other catfish, arrive carrying worms. Deworming during quarantine is one of the most useful things you can do for new fish. This guide covers tapeworms, roundworms (nematodes) including Camallanus, and the medicines that treat each.

How to identify it

Signs to look for

Thin despite eating well. A sunken belly, a "knife-edge" back, or a head that looks too big for the body.
White, stringy or clear poop that hangs from the fish. This is a clue, not proof. It can also mean the fish has not eaten for a while.
A bloated belly in some cases, from heavy worm loads or a blocked gut. See bloated fish.
Red threads sticking out of the vent (the opening behind the belly). This is Camallanus. See our Camallanus guide.
Slow growth in young fish and poor breeding in adults.
Lethargy and death in heavy infections.

The main types of worm

Worm	What it is	Usual treatment
Tapeworms (cestodes)	Flat, segmented worms in the gut. Fish usually catch them from eating live copepods or other small animals.	Praziquantel
Capillaria and other gut roundworms	Thin roundworms that can spread directly from fish to fish. Common in angelfish, discus and zebra danios.	Fenbendazole or levamisole
Camallanus	A red roundworm that sticks out of the vent. Common in livebearers such as guppies.	Levamisole or fenbendazole
Flukes inside the gut	Flatworms related to skin and gill flukes.	Praziquantel

You cannot tell which worm you have just by looking at the fish. A vet can look at a fresh poop sample under a microscope and often identify the eggs or worms. If you can't get a test, many keepers treat for both flatworms and roundworms, one after the other.

When it's a problem

Normal: A small number of worms in an otherwise healthy, well-fed fish. Many fish carry a few without harm.
Early: One fish is a bit thin or has occasional white poop, but eats and swims normally.
Serious: Several fish are wasting, growth has stopped, or you see red worms at the vent.
Emergency: A fish is very thin and has stopped eating, or the belly is swelling fast. Medicated food will not work on a fish that won't eat, so act today.

What to do right now

Test the water for ammonia, nitrite and nitrate. Wasting can also come from poor water or a poor diet.
Check how much the fish really eats. Watch at feeding time. Bullied fish and slow eaters, like many plecos, can be thin because they miss out on food. See feeding and pleco feeding.
Look at the vent with a torch for red threads.
Collect a fresh poop sample if you plan to see a vet. It needs to be checked quickly.
Remove any dead fish at once so others cannot eat it.
Decide where to treat. If your tank has shrimp or snails, plan to treat the fish in a hospital tank.

What to do next

Medicated food or a bath?

Medicated food puts the medicine straight into the gut, where the worms are. It works best while the fish is still eating well. You can buy pre-made dewormer foods, or soak or bind medicine into food following the product's directions.
A bath (medicine added to the water) reaches fish that are not eating well, but less of the drug gets into the gut. Baths also expose everything else in the tank to the medicine.

The main active ingredients

Praziquantel: kills flatworms, including tapeworms and flukes. It does not kill roundworms such as Camallanus or Capillaria. It is generally gentle on fish, plants and filter bacteria, and hobby references describe it as the safest choice in tanks with shrimp and snails. It kills planaria, though, and that die-off can foul the water. See planaria.
Levamisole: kills roundworms, including Camallanus and Capillaria. It is often given as a bath and paralyses the worms so they are passed out. In some countries it needs a vet prescription. Reports on its safety with shrimp and snails are mixed, and some roundworm products warn they are not tolerated by invertebrates. Treat in a hospital tank if you keep them.
Fenbendazole: kills roundworms, and is best given in food. It is toxic to many snails and can kill sensitive shrimp at higher doses. It also kills hydra and planaria. Do not use it in a tank with snails or shrimp you want to keep.
Other products: Some aquarium dewormers combine ingredients, or use different drugs such as flubendazole. Read the label to see what each kills and whether it is safe with invertebrates.

Always follow the product label or your vet's directions for the dose. Fish dewormers sold for aquarium use are not formally approved as medicines in many places, and doses vary between products.

Timing

Most dewormers kill adult worms but not eggs or larvae. Plan a second round about two to three weeks after the first to catch worms that hatch later. Between rounds, vacuum the gravel well and change 30–50% of the water to remove eggs and dead worms.

Cautions for sensitive animals

Shrimp and snails: treat fish in a hospital tank whenever you use fenbendazole, and be careful with any other dewormer.
Plecos and other catfish, loaches, and scaleless fish: can be more sensitive to bath treatments. Start with medicated food where possible, and watch closely for gasping or loss of balance.
Plants: usually tolerate these dewormers, but snails living on them may not.
Filter carbon: remove it while treating, because carbon pulls medicine out of the water. Add fresh carbon afterwards to clear leftovers.
Oxygen: add an air stone during bath treatments.

Deworming wild-caught fish and plecos in quarantine

Wild-caught fish, including many L-number plecos, often carry worms picked up in the wild. Experienced keepers commonly deworm them during quarantine. A typical approach is to let the fish settle and start eating for a week or so, then treat with praziquantel for flatworms and, separately, a roundworm treatment such as levamisole or fenbendazole, each repeated after two to three weeks. Plecos are slow, night-time eaters, so check that medicated food actually reaches them, for example by feeding after lights out. Ask a fish vet if you are unsure.

Products that help

Praziquantel treatment: for tapeworms and flukes. Safest choice around shrimp and snails.
Levamisole treatment: for Camallanus and other roundworms, usually as a bath.
Fenbendazole or flubendazole medicated food: for roundworms. Keep away from snails and shrimp.
Food binder or gel food base: helps medicine stay in food instead of leaching into the water.
Hospital tank with sponge filter: treat fish away from invertebrates.
Gravel vacuum: removes eggs and larvae between treatments.
Activated carbon: removes leftover medicine after treatment.

How to prevent it

Quarantine new fish for 4–6 weeks and deworm during that time, especially wild-caught fish.
Be careful with live foods such as tubifex worms and wild-caught copepods, which can carry worm larvae. Frozen foods carry less risk.
Remove dead fish quickly.
Vacuum the gravel and change water regularly.

Why it happened

Some worms, such as Capillaria, spread directly from fish to fish through eggs in the water and gravel. Others, such as Camallanus and many tapeworms, need a tiny crustacean like a copepod as a go-between, which fish eat. Wild-caught fish and fish from crowded farms often arrive already infected, and worms can spread quickly in a busy tank.

When to get help

Ask an aquatic vet if fish keep wasting after two rounds of treatment, if you are not sure what you are treating, or if you breed fish. A fresh poop exam or a check of a recently dead fish can identify the worm. Many other problems, including fish TB, also cause wasting.

In the game

Multiple Tank Syndrome doesn't have an internal worms card yet. The "Quarantine tank" card is the best defence.