Hydra vulgaris in freshwater aquariums

Maintaining a perfect home for your fish and shrimp is important in their survival. If not maintained, destructive pests can live and reproduce very rapidly.

What are Hydras?

Hydras are a genus of fresh-water organisms that belong to the phylum Cnidaria, related to jellyfish. They belong to the class Hydrozoa and named after a sea snake in Greek mythology. They resemble plants but are tiny animals!

Hydra vulgaris is a soft-bodied polyp that only grows to 30mm, are pests and can accumulate in your aquatic system. They appear brown, green, or white and characterised by their tube-like body with tentacles and a small foot to anchor themselves to a substrate. Even with an anchored foot, they can be seen moving to find a preferable spot in your fish’s aquarium. Hydrozoa organisms have specialised cells called myocytes which are involved in defence, feeding and attachment. The tentacles unlike some jellyfish will not harm humans but can be lethal to your fish. The tentacles contain stinging cells called nematocysts located in the epidermal cells that can immobilise prey and eventually draw them into the digestive cavity to consume. When the Hydra becomes threatened, they can retract their body making them appear circular.

Even without a brain, respiratory or circulatory system, they can reproduce asexually very rapidly by budding and eventually break off into polyps to form a colony of Hydras. They have a gastro-vascular cavity which means they only have one opening for excreting waste and consuming food, located in the middle of their tentacles.


How do they contaminate fish aquariums, and should you be worried?

Unfortunately, they sometimes are accidentally introduced when live plants are added into fish aquariums or even seen hitchhiking in the water that you bring your fish home in from the pet shop. They can survive on any surface in your aquarium. Before introducing plants into your aquarium thoroughly clean them. A pre-treatment can be used consisting of mild bleach or hydrogen peroxide.

Equipped with toxic tentacles and strict carnivores, they are ready to prey on your fish when they get too close. Their main victims are larvae including shrimp and fry but can also be seen munching on smaller fish. They sting shrimp and fry which causes stress, damages their immune system, and makes them lose their appetite. Larger fish are not affected by Hydras, but the infestation should not be ignored.

They are more common in aquariums where live food is fed to your fish. This encourages the Hydra to grow and reproduce. Using pellet or flake food can help reduce them from breeding. Ensure your aquarium is always clean and avoid overfeeding your aquarium residents.


What to do if you find Hydra in your aquarium?

Once identified, action must be taken to remove them quickly. Firstly, assess how severe the infestation is and how drastic the measures you must take to remove Hydra from your aquarium. It can be challenging to remove them, so here are three methods to combat these intruders.


  • Manual removal:


Luckily if only a minor infestation has occurred you can physically remove them. If seen on the sides of the aquarium, carefully use a sponge to wipe them off, but be sure to remove the whole Hydra body as even a small part left behind can reproduce into more polyps. This process is beneficial if your whole aquarium is not infested, otherwise, another  removal method may be more effective.


  • Hydra predators/biological removal:

Hydras can make a great protein meal for your fish and snails and is a much less disruptive approach to remove them. Introducing mollies, three spot gouramis and paradise fish to your tropical aquarium could possibly tackle your infestation problem. However, do not add these predators to your tank if you also keep baby shrimp, as they will also become a meal for your new fish. Some people have found that introducing the common pond snail into your aquarium is also a natural way of pest control. They may consume the Hydra, but they also consume waste products which reduce microscopic organisms that the Hydra feed on and use to rapidly reproduce.


  • Chemical removal:


This should be the last resort to removing Hydra in your aquarium, but it is extremely fast and effective. Chemical treatments can be purchased at a fish retailer to eliminate them.

When using chemical treatments be sure to remove any snails you wish to keep as it can be lethal to them too. Hydra cannot survive beyond 40°C so if you increase the water temperature for a few hours they will eventually die. After this, be sure to siphon the gravel, clean the filter and replace 50% of the water in the aquarium.

Before adding any chemicals or increasing the temperature of your aquarium, live plants and fish should be removed. Once treatment has been carried out, check the water quality is safe for your fish and plants to return home.

You can also use a copper-based anti-parasitic medication, but the fish may not tolerate this fully and it can kill live plants in your aquarium and any fry. Some people find diluted de-wormer containing fenbendazole is also successful.

It is always best to talk to your local fish retailer on the best chemicals to use before adding them into your aquarium. If your aquarium does become infected with Hydra do not panic but think about what approach best suits the infestation.