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How to Get Rid of Hydra in Your Aquarium

Have you spotted a miniature tentacle monster in your freshwater aquarium? Not to worry – it’s a fascinating freshwater creature called hydra that is relatively easy to deal with. Keep reading as we talk about what is hydra and a few natural methods of removing them without harming your animals, plants, or beneficial bacteria.

What is Hydra?

These tiny, freshwater organisms of the genus Hydra are the distant relatives of jellyfish, corals, and sea anemones. Growing up to 0.4 inches (1 cm), they range in color from translucent white to green to light brown. Much like a sea anemone, hydra has a stalk or foot that attaches to surfaces (like plants, hardscape, or glass) and a mouth at the other end that is surrounded by long, wispy tentacles. These tentacles have stinging cells that are used to paralyze and catch their prey.

Scientists have long been interested in hydra because of their “immortal” cells and powerful regenerative abilities. If a hydra is split into pieces, each fragment regenerates to become a new, individual hydra. They can also reproduce asexually by producing buds or sexually by creating eggs.

green hydra with bud

Green hydra (Hydra viridissima) has a unique, symbiotic relationship with photosynthetic Chlorella algae, which is responsible for its green pigment.

How did hydra get in my fish tank? In our experience, we’ve noticed that hydra often lays dormant in fish tanks for many months, but then the population blooms when you start heavily feeding baby brine shrimp. It’s possible that the hydra hitchhiked from aquarium decorations, aquatic plants, rocks, or driftwood that came from an infected tank. Hydra can also be introduced if you collect live foods, plants, or hardscape from the wild.

Are hydra dangerous to humans? No, the stinging cells are too weak to affect humans. If you try to touch them, they quickly retract their tentacles and ball up to avoid predation from larger animals.

Are hydra bad for aquariums? Hydra are ambush hunters that like to eat microworms, insect larvae, and tiny crustaceans (e.g., cyclops, daphnia, scuds, and baby brine shrimp). In our experience, they are a natural part of the aquarium ecosystem and do not seem to greatly impact baby fish and shrimp populations. Adults are too big to be eaten, and fry have a strong flight response that causes them to jerk away from any stimulus, like a stinging tentacle.

How to Get Rid of Hydra

Unless you have a steady hand and a very small population of hydra, manual removal is generally not advised. If you accidentally break off any pieces of the hydra, they will grow into new hydra. Instead, we first recommend that you decrease the amount of food going into the tank. When hydra don’t get enough food, the majority of them will starve to death and eventually disappear. Consider target feeding the fish or using feeding dishes for shrimp to prevent the food from spreading throughout the aquarium. Also, regular water changes and gravel vacuuming to remove excess food will help decrease the population to unnoticeable levels.

Another natural removal method is to add predators to eat the hydra. You can try just about any omnivorous or carnivorous fish that is small enough to notice the hydra – such as guppies, mollies, betta fish, paradise fish, and gouramis. If the fish do not seem to consume the hydra, try reducing feedings to whet their appetites.

yellow molly fish

Aquariums with adult fish and snails rarely get large hydra populations because hydra is a convenient source of live food.

Hydra are particularly prominent in fry grow-out tanks and shrimp-only aquariums because we purposely overfeed them with hydra-sized foods like baby brine shrimp or powdered fry food. Plus, we usually remove all would-be predators that are big enough to eat both fry and hydra. Luckily, you can add snails (like ramshorn, pond, and spixi snails) that are happy to consume hydra but are too slow to go after baby fish and shrimp. Plus, snails do a great job of cleaning up any excess food that is not eaten by the fry.

As a last resort, consider using chemical treatments to kill hydra, but do your research to make sure that these methods do not harm the other inhabitants of the aquarium. You can also consider treating live plants and decor before adding them to your aquarium as long as the treatment will not adversely affect the plants and aquatic animals. At our fish store, we like to use Merck's Safe-Guard liquid dewormer for goats (suspension 10%, 100 mg/ml), which contains fenbendazole as the active ingredient. Make sure to shake the bottle vigorously before treating the aquarium with 1 drop of medication per gallon of water. The medication works very quickly within 24 hours and has been safe to use with snails, shrimp, fish, plants, and beneficial bacteria in our experience. After three days, do a 50% water change and redose the tank if needed. 

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