Hydra in Aquariums: Identification & Control
Spot hydra by their stalk and stinging tentacles, understand how they hitchhike in and bloom on overfeeding, and control them without harming shrimp.
Overview & Identification
Hydra are tiny freshwater cnidarians (phylum Cnidaria, class Hydrozoa) — relatives of jellyfish and sea anemones. Each is a stalked polyp: a tubular, radially symmetric body anchored by a basal disc (foot), topped with a ring of one to twelve thin, mobile tentacles around the mouth. Extended, a hydra reaches up to about 10 mm. Unlike a worm, it stays attached to one spot and waves its tentacles to catch passing prey.
- Stalk anchored to glass, plants or decor by a basal foot
- Crown of fine tentacles (1–12) that retract when disturbed
- Up to ~10 mm extended; white, tan or greenish
- Reproduces asexually by budding, so a few become many fast
Where They Come From
Hydra hitchhike into the tank on aquatic plants, rocks, driftwood and decorations from infected or wild-collected sources, and they can lie dormant for many months before appearing. They surge into a visible population when there is plenty of tiny food in the water — classically after you begin heavily feeding baby brine shrimp or powdered fry food.
Are They Harmful?
Hydra are carnivores armed with cnidocytes — stinging cells whose nematocysts discharge neurotoxins on contact and can subdue prey larger than themselves. To adult fish they are harmless, but in fry and dwarf-shrimp breeding tanks they can sting and kill newly hatched fry and shrimplets. Many keepers note that fry with a strong flight response often escape, so the main threat is to the smallest, slowest newborns. A hydra bloom is, above all, a sign of overfeeding.
Control & Removal
Cut the food supply first: target-feed, do regular water changes, and the population usually crashes on its own. Manual removal helps for a few visible polyps. Some keepers add predators such as guppies, mollies, bettas or gouramis. For stubborn blooms, fenbendazole (around one drop of medication per gallon, with a 50% water change after about three days) reliably wipes hydra out.
Prevention
Feed sparingly, especially with fine fry foods, and siphon out leftover food and detritus. Inspect, rinse or dip new plants and decor before adding them, since a single dormant hydra is enough to seed a tank when feeding ramps up.
Common Mistakes
- Treating with fenbendazole while prized snails are still in the tank
- Heavy fry feeding without target-feeding, which fuels the bloom
- Scraping hydra off without cutting feeding — they simply return
- Skipping the post-treatment water change