Hydra in Aquariums

Hydra are tiny freshwater predators with stinging tentacles. They thrive on overfeeding and threaten fry and shrimp; control by cutting feeding and targeted treatment.

Identification

Hydra are small freshwater cnidarians, relatives of jellyfish and corals, that grow up to about 10 mm long when extended. They have a tubular body anchored to a surface at one end, with a ring of slender tentacles around the mouth at the other. They are solitary, carnivorous animals and can appear suddenly on glass, plants and decor.

How they feed

Hydra tentacles carry specialised stinging cells called nematocysts, which fire dart-like threads containing neurotoxins on contact with prey. They extend the tentacles and wait for small prey such as tiny crustaceans (Daphnia, Cyclops, copepods), worms and larvae to touch them, then paralyse and engulf the catch. A hydra can stretch its body to digest prey more than twice its own size.

Risk to fry and shrimp

Because they capture small crustaceans and larvae, hydra are a particular threat in fry grow-out and shrimp tanks. They bloom precisely in these tanks because keepers deliberately add fine foods like baby brine shrimp and powdered fry food, which also feed the hydra. Their potent stinging cells can paralyse very small prey, so newly hatched fry and shrimp larvae are at risk.

How to remove them

Reduce feeding: when hydra are deprived of food, most will starve and die back.
Use target feeding and feeding dishes to avoid scattering food.
Add predators that eat hydra, such as guppies, mollies, bettas, paradise fish and gouramis, or slow snails like ramshorn and pond snails.
As a chemical option, a fenbendazole-based dewormer is commonly used and works quickly; follow tested dosing and do a water change afterward.

Prevention

Feed sparingly, especially in shrimp and fry tanks.
Quarantine and rinse new plants and decor that may carry hydra.
Keep up regular water changes and substrate cleaning.
Watch new tanks closely so any hydra is caught early.