Planaria in fish tank: how to tell them from harmless white worms, and how to get rid of them

Most white worms in a fish tank are harmless detritus worms or nematodes that feed on leftover food. Planaria are the exception: flat worms with an arrow-shaped head that glide on the glass. They don't harm fish, but they eat shrimp eggs and baby shrimp, so shrimp keepers should act.

Whichever worm you have, the first fix is the same: feed less and clean up waste. Only use a dewormer once you're sure it's planaria, and protect your snails first, because the usual treatment kills them.

How to identify it

Look closely with a magnifying glass or a phone camera on zoom, against the glass.

Worm	Shape	How it moves	Harmful?
Planaria	Flat, 3 to 15 mm, with a triangular or arrow-shaped head and two eye spots that look "cross-eyed"	Glides smoothly over glass like a tiny snail	Yes, to shrimp eggs, shrimplets, snail eggs and very small fry
Detritus worms	Thin, round, thread-like, white to reddish, up to a few cm long	Wriggles and thrashes; lives in the gravel, sometimes swims in open water	No; a sign of too much waste
Free-living nematodes	Tiny, very thin, white	Whips back and forth in an S-shape on glass or in the water	No
Rhabdocoels (other flatworms)	Small and flat, but with a rounded head	Glides like planaria	Generally not considered a problem

The bait test: put a small piece of raw meat, fish or a pellet in a dish on the bottom overnight. Planaria gather on it, which makes them easy to spot and confirm.

Not worms in the water: red or white threads hanging from a fish's vent are internal parasites such as camallanus, which need a different treatment.

When it's a problem

Normal: a few detritus worms in the gravel. Most tanks have them.
Early problem: lots of detritus worms or nematodes on the glass, especially after feeding. This means too much food.
Serious: planaria in a shrimp tank, or shrimplet numbers dropping. Planaria multiply quickly when food is plentiful, and can split in two to reproduce.
Emergency: worms are rarely an emergency. But if fish are also gasping, test ammonia: worm booms often go with a dirty tank.

What to do right now

Work out which worm you have, using the table and bait test.
Cut feeding to what's eaten in a minute or two. For shrimp, remove leftovers after two hours.
Vacuum the gravel thoroughly, especially under decor and in corners.
Rinse the filter sponge in old tank water.
Do a 25 to 30% water change with dechlorinated, temperature-matched water.
If it's only detritus worms or nematodes, stop there. They'll fade as the food runs out.

What to do next

For detritus worms and nematodes: keep feeding small and vacuuming weekly for a few weeks. No medicine needed. Fish will eat many of them.

For planaria, week 1: trap and reduce. Set a bait trap or a dish with meat each night and remove it with the planaria every morning. Traps rarely clear an outbreak alone, but they reduce numbers and confirm what you have.

For planaria in a shrimp tank, week 1 to 3: treat with a dewormer. The common options are:

Fenbendazole (sold as a dog and livestock dewormer). Widely used by shrimp keepers and usually effective.
Flubendazole, a related dewormer with similar effects.
Planaria treatments sold for shrimp tanks, some based on betel nut extract. Follow the label.
Praziquantel is used against flatworms and some keepers try it, but results against planaria are mixed.

Important cautions:

Snails: fenbendazole and flubendazole kill many snails, including ramshorns and bladder snails, and sometimes nerites and mystery snails. Move any snails you want to keep to another tank first. Some keepers report that the drug lingers in the tank for weeks to months.
Shrimp: most keepers report shrimp come through a correct dose fine, but crustaceans are sensitive to these drugs. In a lab study, fenbendazole was most toxic to water fleas (Daphnia, a tiny crustacean) of all the organisms tested. Never overdose, and consider treating a valuable colony in a separate tank.
Catfish, plecos and loaches: a report from the National Aquarium in Baltimore linked fenbendazole in food to illness and deaths in some bottom-dwelling freshwater fish, including Corydoras catfish. Take care if you keep these fish, and ask a vet if unsure.
Dosing: products differ in strength (often 10% or 22% fenbendazole), and sources give very different doses. Follow the product label or a vet's advice, and measure carefully.
Remove activated carbon from the filter during treatment, as it absorbs medicine.
Dead worms rot. Vacuum them up after 24 to 48 hours and do a water change. Test ammonia daily.
Finish the course. Planaria eggs may survive, so some keepers repeat the treatment after one to two weeks.
After treatment, run activated carbon for a few days and do large water changes before returning snails.

Products that help

Planaria trap: a small baited tube or box that worms can crawl into. Reduces numbers and confirms the ID.
Fenbendazole or flubendazole dewormer: the most common treatment for planaria and hydra in shrimp tanks. Kills snails.
Shrimp-tank planaria treatment: an alternative designed for shrimp tanks. Read the snail warnings.
Gravel vacuum: removes the waste worms feed on.
Magnifying glass: to tell planaria from detritus worms.
Activated carbon: to remove medicine after treatment.
Spare tank or tub: to hold snails during treatment.

How to prevent it

Feed less, and remove leftovers.
Vacuum the gravel regularly.
Quarantine new plants, shrimp and snails. Planaria hitchhike on plants, moss and in shop water.
Never pour shop or seller water into your tank.
Remove dead shrimp and snails quickly; planaria feed on them.

Why it happened

Worms are carried in on plants, moss, decor, live food or shop water. They stay in small numbers until there's spare food, then multiply. A worm boom almost always means too much food or waste in the tank.

When to get help

Ask for help if you can't tell which worm you have, if worms are coming out of fish rather than living on the glass, or if fish, shrimp or snails die after treatment. A shrimp club or experienced shop can identify worms from a clear photo or video. A vet who treats fish can advise on dewormers.

In the game

Multiple Tank Syndrome doesn't have a planaria card yet. The closest is the "Levamisole dewormer" product, which saves fish from "Camallanus worms".

Sources

Aquarium Science. Pests.
Fish Tank Advisor. Planaria worms in the aquarium: removal and prevention guide.
Wagil M and others (2015). Toxicity of anthelmintic drugs (fenbendazole and flubendazole) to aquatic organisms. Environmental Science and Pollution Research 22: 2566–2573.
Veterinary Information Network (abstract). Morbidity and mortality associated with fenbendazole use in teleost fish at the National Aquarium, Baltimore.