Hydra in fish tank: how to identify them, protect fry and shrimp, and get rid of them

Hydra are tiny freshwater relatives of jellyfish that stick to glass and plants and catch food with stinging tentacles. They're harmless to adult fish and adult shrimp, but they can catch and kill fish fry and baby shrimp, so act if you're breeding.

Hydra multiply when there's lots of tiny food in the water. Cutting back feeding is the first fix. If they persist in a breeding tank, a dewormer such as fenbendazole usually clears them, but it can kill snails.

How to identify it

Size: usually 2 to 15 mm (up to about 1/2 inch) when stretched out.
Shape: a thin stalk stuck to a surface at one end, with a ring of 4 to 12 long, thin tentacles waving from the other. They look like a tiny palm tree or an upside-down dandelion clock.
Colour: white, cream, brown or bright green. Green hydra carry algae inside them.
Behaviour: they shrink into a small blob when touched or when the lights come on, then slowly stretch out again.
Where: on the glass (often the lit side), plants, decor and filter parts.
Reproduction: small buds grow from the side of a hydra and break off as new ones, so numbers can rise fast.

Not hydra: flat gliding worms are planaria; tiny thrashing threads are nematodes or detritus worms. Rare look-alikes include bryozoans and stentors, which a magnifying glass or microscope can tell apart.

When it's a problem

Normal: a few hydra in a tank with only adult fish. They're harmless there and will often fade on their own.
Early problem: numbers rising on the glass, especially after you started feeding baby brine shrimp or powdered fry food.
Serious: hydra in a tank with fry, eggs or baby shrimp. A study of bluegill sunfish found that hydra living on water plants caught and ate fish larvae, and that many larvae that escaped later died from the stings.
Emergency: hydra themselves aren't an emergency, but fry losses can be. Move fry or shrimplets to a clean, hydra-free tank or container if you can.

What to do right now

Confirm it's hydra with a magnifying glass or zoomed phone photo.
Cut feeding, especially baby brine shrimp, microworms and powdered foods. Hydra live on the same tiny food.
Vacuum the gravel and do a 25 to 30% water change with dechlorinated, temperature-matched water.
Wipe the glass to remove as many hydra as you can, and siphon them out.
Move fry or shrimplets to a separate hydra-free container if numbers are high.

What to do next

Weeks 1 to 3: starve them. In tanks with adult fish only, cutting back food and keeping the tank clean is often enough. Hydra shrink and disappear when there's nothing to catch.

In breeding and shrimp tanks: treat with a dewormer. Fenbendazole (a dog and livestock dewormer) and related drugs such as flubendazole are commonly used and usually work within days.

Snails: these drugs kill many snails, especially ramshorns and bladder snails, and sometimes others. Move snails you want to keep before treating. Some keepers report the drug lingers in the tank for weeks to months.
Shrimp: most keepers report adult shrimp survive correct doses, but crustaceans are sensitive. In laboratory tests, fenbendazole was most toxic to water fleas (a tiny crustacean). Never overdose.
Catfish and loaches: a National Aquarium report linked fenbendazole given in food to illness and deaths in some bottom-dwelling freshwater fish, including Corydoras. Take care with catfish, plecos and loaches, and ask a vet if unsure.
Dose: products differ in strength and published doses vary widely. Follow the product label or a vet's advice, and measure carefully.
Remove activated carbon during treatment.
Clean up afterwards: after two or three days, vacuum dead hydra, do a large water change, and test ammonia. Run fresh carbon for a few days before returning snails.
Finish the course: watch for new hydra over the next two weeks, and repeat once if needed.

The heat method, with caution. Some breeders raise the temperature of a tank with no animals in it to around 40°C (104°F) for a short time to kill hydra. Only do this in a tank you have emptied of all fish, shrimp and snails. It can kill plants and filter bacteria, so you may need to re-cycle the tank afterwards. Heaters, glass and silicone can also be stressed by very hot water. Never do this with livestock in the tank.

Natural predators. Some gouramis and paradise fish eat hydra, and some pond snails graze on them, but results are hit and miss, and these fish will also eat fry and shrimplets.

Products that help

Fenbendazole or flubendazole dewormer: the usual hydra treatment in breeding tanks. Remove snails first.
Magnifying glass: confirms the ID before you treat.
Gravel vacuum: removes the waste and uneaten food that feed hydra's prey.
Algae scraper: wipes hydra off the glass.
Activated carbon: removes medicine after treatment.
Spare tub or tank: holds snails, fry or shrimplets during treatment.
Plant dip: for new plants, to kill hitchhikers before they go in. Check it's safe for the plant species.

How to prevent it

Feed fry small amounts, often, and remove what isn't eaten within a few minutes.
Hatch and rinse live foods well before feeding.
Quarantine new plants, moss and shrimp, and don't add shop water.
Keep fry tanks clean, with regular small water changes.

Why it happened

Hydra usually arrive on plants, moss, live food or in water from another tank. They stay small and few until there's lots of tiny food in the water, such as baby brine shrimp or the tiny animals that thrive on leftover food. Then they bud and multiply.

When to get help

Ask for help if you can't confirm what you're seeing, if fry or shrimp keep disappearing after treatment, or if fish become ill after a dewormer. A shrimp or killifish club, an experienced breeder, or a vet who treats fish can advise.

In the game

Multiple Tank Syndrome doesn't have a hydra card yet. The closest is the "Pleco breeding cave" tank, where protecting fry matters most.

Sources

Elliott JK, Elliott JM, Leggett WC (1997). Predation by Hydra on larval fish: field and laboratory experiments with bluegill (Lepomis macrochirus). Limnology and Oceanography 42(6): 1416–1423.
Aquarium Science. Pests.
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.