Hydra (freshwater pest) (Hydra vulgaris) care guide

Group

Other invertebrates

Other names

Hydra, Common hydra, Hydra attenuata, Hydra magnipapillata

Family

Hydridae

Origin

Widespread in fresh waters worldwide, including Britain

Water

freshwater

Maximum size

3 cm

Care

Hydra are tiny freshwater relatives of jellyfish and anemones. Hydra vulgaris is a slim, pale or greenish polyp 1–3 cm long and about 1 mm wide, with 4–12 thin tentacles round the mouth, standing on a sticky disc-shaped foot. It is a pest in shrimp and fry tanks and should not be sold as livestock. How it arrives: on aquarium plants, in pond water, with live foods such as daphnia or cyclops, and in water from other tanks. Numbers explode when there is plenty of food, especially in tanks fed heavily with baby brine shrimp or powdered fry food. Identification: look on the glass, plants and decor for small stalked polyps with tentacles spread out, which shrink into a tiny blob when touched. They can let go and drift, or move by somersaulting along the glass. Harm: they catch prey with stinging tentacles. Hydra feed on tiny animals and can catch and kill fish fry and newborn shrimplets, so they are a nuisance in breeding tanks. They are harmless to adult fish and shrimp. Removal and prevention: rinse or dip new plants and avoid adding pond water. Reduce feeding, because hydra starve when food is scarce. Some gouramis, mollies and pond snails eat them. Shrimp keepers often use fenbendazole or flubendazole treatments, which kill hydra and planaria without harming most shrimp, but they can kill snails, so remove snails first and follow the product instructions. Even small pieces can regrow into a new hydra, so scraping them off is not enough on its own. Copper: copper-based medicines and some plant fertilisers are toxic to shrimp, snails and most invertebrates. Never dose them in a tank holding this animal, check the label of any treatment or fertiliser, and use a separate hospital tank for fish. Sources: Wikipedia (en.wikipedia.org/wiki/Hydravulgaris). Checked October 2026.

Diet

Predator of tiny animals such as water fleas, copepods, baby brine shrimp, fry and newborn shrimp, caught with stinging tentacles. It should not be fed or kept in a breeding tank.

Breeding

Mostly reproduces by budding: a small new hydra grows from the side of the parent and drops off, which is how numbers rise fast when food is plentiful. It can also reproduce sexually, the fertilised egg forming a tough shell that waits for good conditions. Cut pieces can regrow into whole animals. Stem cells let it keep renewing its body, and it is described as biologically immortal, though this is disputed.

Compatibility and tank mates

Harmful to fry and newborn shrimp; harmless to adult fish, shrimp and plants. Eaten by some gouramis, paradise fish, mollies and some snails.

Last updated 8 October 2026. Community-edited care guide on Aquatic-Auctions.