Camallanus worms: red worms coming out of your fish, and how to get rid of them

Red or pink threads sticking out of a fish's vent (the opening behind the belly) are camallanus worms, parasitic roundworms that live in the gut. If one fish has them, assume the whole tank does, and treat every fish with a dewormer that works on them: levamisole or fenbendazole.

The adult worms hook into the gut wall and feed on blood, which is why they're red. The threads you see are adult females releasing live larvae into the water. The Merck Veterinary Manual calls this "red worm disease". The species most often found in aquariums is Camallanus cotti. It's common in livebearers (guppies, platies, swordtails, mollies) and also infects cichlids, plecos and many other aquarium fish (UF/IFAS).

How to identify it

One or more red threads at the vent. They may pull back inside when the fish is disturbed, so look with a flashlight when the lights first come on.
Weight loss: a thin or "hollow" belly, even though the fish eats.
A swollen or red vent, lethargy, clamped fins, hiding.
Light infections may show no signs at all.

Easily confused with:

Red poop after feeding bloodworms or red food. Poop drops off; worms stay attached.
A prolapsed gut: pinkish tissue, not threads.
Other internal worms or gut flagellates, which cause weight loss without visible worms.

A vet can confirm it.

When it's a problem

Early: a single tiny thread now and then, fish still eating and active. Treat now, while it's easy.
Serious: several worms clearly visible, fish losing weight, swollen vent, clamped fins.
Emergency: fish wasting badly and dying, especially small fish and livebearer fry. Aquarium Science reports livebearers suffer higher losses.
Normal, don't worry: a red strand that drops off soon after a meal of red food. That's poop.

Any real camallanus worm is a problem for the whole tank, even if only one fish shows it.

What to do right now

Check every fish in the tank. If one has worms, the tank has worms.
Don't move fish to other tanks until you've treated. You'll spread it.
Vacuum the gravel well and do a 25–30% water change. Larvae live in the water and on the bottom.
Remove activated carbon from the filter, or it will absorb the dewormer.
Plan for shrimp and snails. Many dewormers harm them, especially at higher doses. Move them out if you can.
Get the right drug: levamisole or fenbendazole. Many "general cure" products and praziquantel don't work on camallanus.

What to do next

The drugs that work

Both the Merck manual and UF/IFAS list levamisole and fenbendazole for camallanus. They're livestock and pet dewormers; some are sold in aquarium versions. Depending on where you live, you may need a vet to get them.

Levamisole paralyses the worms so they let go and pass out of the fish. It's used as a bath in the water or mixed into food.
Fenbendazole is usually given in food. It works more slowly, by starving the worms.

Products and strengths vary widely. Follow the label or your vet's directions for your product, and measure your real water volume.

Bath or food? Experts disagree

UF/IFAS gives both bath and food-based levamisole methods. Aquarium Science argues that medicine in the water mainly paralyses worms poking out of the fish, and that medicated food is what reaches worms inside. Many experienced breeders use a bath and medicated food together, or follow a bath with medicated food. If a bath alone fails, switch to food, or ask a vet.

Why one dose isn't enough

Dewormers kill adult worms. Larvae already in the gut, or in the tank, can survive and grow. UF/IFAS describes repeating treatment after 2–3 weeks. Aquarium Science says larvae can survive about 3 weeks outside a fish, and worms take 3–4 weeks to mature inside. That's why most protocols repeat at least once or twice, a few weeks apart. Finish the full course, and keep checking vents with a flashlight for at least 2 months after the last worm.

How it spreads, and why you treat everyone

In nature, larvae are eaten by tiny crustaceans called copepods, and fish get infected by eating those copepods. UF/IFAS describes this indirect life cycle. But a 2002 study by Levsen and Jakobsen found C. cotti can also infect fish directly, with no copepod, and that aquarium populations can keep going for months in tanks with no copepods. The direct route is less efficient, but in a home tank it's enough. So:

Treat the whole tank at the same time, including fish that look healthy.
Vacuum the gravel the day after each treatment to remove expelled worms and larvae.
Add an air stone during levamisole baths. Some keepers report fish act sluggish or oxygen drops during treatment.

Cautions

Shrimp and snails: deaths have been reported with both drugs, especially at higher doses and with fenbendazole. Remove them, or at least accept the risk knowingly.
Plecos and other catfish, loaches: generally tolerate levamisole, but watch them closely after dosing.
Live plants: usually fine with these drugs.
Filter bacteria: test ammonia and nitrite during and after treatment, especially if many worms die off.
Activated carbon: keep it out of the filter during treatment.
Resistance: some keepers report worms that don't respond to one drug. If one fails after a full course, ask a vet about switching.

Products that help

Levamisole dewormer: paralyses the worms so they pass out of the fish. Use as a bath or in food when you've seen worms. May need a vet.
Fenbendazole dewormer: given in food; slower, starves the worms. Useful on its own or after a bath.
Gravel vacuum (siphon): removes expelled worms and larvae from the bottom. Use the day after each treatment.
Air stone and air pump: keeps oxygen up during levamisole baths.
Ammonia and nitrite test kit: checks that the filter is coping during treatment.
Flashlight: for checking vents when the lights first come on.
Quarantine tank: a separate tank for new fish, so worms don't reach your main tank.

Before trouble starts, keep a quarantine tank, a gravel vacuum, separate nets for each tank, and a test kit on hand.

How to prevent it

Quarantine new fish for 4–6 weeks and watch their vents. Many breeders deworm all new livebearers in quarantine as routine.
Avoid wild-collected live foods unless you know they're clean.
Use separate nets per tank, and don't move fish or water between tanks casually.

Why it happened

A new fish carried worms in. Livebearers from large farms are a common source, and light infections are invisible.
Live foods that contain copepods (some pond-collected live food) can carry larvae, per UF/IFAS.
Fish moved between tanks without quarantine.

When to get help

See a vet if you can't get levamisole or fenbendazole legally, if worms persist after two full courses, or if fish are wasting badly. A vet can examine a fecal sample or a dead fish to confirm the worm and rule out others. Honest limits: badly infected fish may already have gut damage, and some will die even after the worms are gone.

In the game

In Multiple Tank Syndrome, the Camallanus worms card ("red worms from the vent, passed fish to fish") kills 2 fish cards at random in the hit tank. The Levamisole dewormer card or the Medicine basket saves the tank.