A Hydra is a soft-bodied polyp of the Phylum Cnidaria, one of the most ancient animal groups in evolutionary terms which includes corals, jellyfish, hydras, and myxozoans. A Hydra has a tubular body with a sticky foot at one end and a dozen tentacles at the other. Like jellyfish, these tentacles possess stinging cells that allow the Hydra to immobilize their prey. They primarily eat small aquatic organisms such as cyclops, daphnia, and other aquatic crustaceans, but can catch and ingest small fish.
Hydras are sometimes accidentally introduced into freshwater aquariums when adding plants. A Hydra has no brain, no circulatory or respiratory system, nor even any musculature, yet it poses a real danger to small freshwater aquarium fish.
Although a small pest, measuring just an inch or less, Hydras are capable of killing and eating most fish fry and even small adult fish. Furthermore, they asexually reproduce rapidly by producing buds that grow into new Hydra, break off, and go out on their own. Although they generally stay in one spot, a Hydra is quite capable of uprooting and moving around; all they have to do is release their foot and float to a new spot. Hydra can also slowly inch along a substrate by attaching and releasing their tentacles and foot alternatively.
Given their reproductive capabilities, their capacity to relocate, and their ability to eat prey several times their size, it's clear why Hydras are not welcome in a freshwater aquarium. Prevention is key.
Once Hydras are introduced to an aquarium, they are difficult but not impossible to eradicate. If you are fortunate to have a very small infestation, you may be able to physically remove the Hydras with deep cleaning. Artificial plants and rocks with attached Hydras can be removed from the tank and soaked in a 10 percent bleach solution for 10 to 15 minutes, then scrub and rinse with plain water and allow them to air dry.
A less disruptive way to eliminate Hydras is to add in fish that eat them. Three Spot (Blue) Gouramis are particularly voracious consumers of Hydra. Paradise fish and Mollies are also very fond of eating Hydra. Even pond snails will gobble them up.
If adding fish or snails isn't an option for you, heat can also be used. However, you will have to remove your fish. Once the fish have been temporarily relocated, increase the temperature of the water up to at least 104 degrees Fahrenheit (40 Celcius) for about two hours. Turn the heat back down, vacuum the gravel to remove any Hydras that have fallen to the bottom of the tank, and perform a large water change (approximately 50 percent) to remove the hot water. Make sure the temperature has dropped to the previous level before reintroducing the fish.
Chemical or medicinal bath treatments are available that will eradicate Hydra, but keep in mind that they are often harmful to live plants, snails, and even the fish themselves. Some medications can also harm your beneficial bacteria in the biofilter, so these treatments should only be considered as a last resort.
Among the chemical options are copper sulfate and potassium permanganate, which can be purchased at your local fish store. Follow dosing instructions on the product label. Many of the anti-fluke medications are also effective against Hydra, particularly those containing Formalin, such as ParaGuard.
Hydras do not spontaneously grow; a Hydra is a stowaway that is always introduced from an external source. Once in place though, having plenty of food sources will encourage their speedy growth. Therefore maintaining a clean tank and avoiding overfeeding will prevent any Hydras that do come in from rapidly multiplying, giving you time to address the problem.
Carefully inspect any new live plants before adding them to the aquarium to help avoid accidentally bringing in a Hydra. An even more proactive approach is to soak live plants for five to 10 minutes in a quart of water with a tablespoon of dissolved alum, a common pharmacy ingredient used in pickling and canning vegetables. This can be done for new rocks as well. Following the soak, rinse the objects well with plain water.
Live foods are great for conditioning fish for breeding, and to provide good nutritional balance. However, if your live foods are collected directly from freshwater ponds or streams, there is a considerable risk of collecting a Hydra at the same time. Using frozen foods or growing your own live foods omits that risk.