General Eel Care by Jonathon Bresolin, DVM

General Information:
Eels are a type of fish characterized by their long, serpentine bodies. Their caudal and anal fins, as well as their dorsal fins, are merged into a continuous fin that extends along around 2/3rds of their body. While there are 22 families of eels, most pet eels fall into the Anguillidae family, which includes the classic Moray eels. There are 16 species within the Anguillidae family, the most common of which include the Asian, American, and Japanese eels. Eels live, primarily, in freshwater systems and migrate into seawater to spawn. Eels in the wild can live up to 20 years.

In order to support their ability to live in both seawater and freshwater, eels have the ability to control their own ingestion of seawater and alter it before it reaches the stomach, preventing dehydration. They also can adjust chloride levels of the cells in their gills, adapting them to withstand the harsh change in environment.

Like all freshwater fish, eels are exposed to dramatic temperature variations during seasonal changes. In order to stabilize the blood and support brain function in colder temperatures, eels modify the amount of glucose that is circulating through their body. This allows them to adapt to and survive winter weather conditions, including almost freezing water temperatures.

Tank Selection:
Eels, despite being relatively sedentary creatures, require a large volume of water to offset their waste production. A minimum of a 60 gallon tank is recommended, even with the smaller species.  A large amount of “fixed” cover is also required. Eels primarily exist in hiding spaces in rocks and cover during the day, and this cover must be stable, in order to prevent it from shifting/falling and crushing the eel. Make sure to settle cover into the sediment, to ensure that it remains steady and in position.
A tank with a cover is a must. Eels are escape artists, and will systematically explore and test the boundaries of their enclosure at night. They are well known for jumping out of tanks. If you find your eel on your floor, don’t lose hope. Even dry and stiff eels can potentially be restored to life. Pick them up with a damp washcloth, rinse them off with tap water, and return to the tank. Monitor the eel for recovery—they should return to normal within an hour, if they are capable of doing so.
In regards to tank-mates, a well-fed eel rarely attacks other fish. However, many eels, especially the Snowflake Eel (Echidna nebulosa) preferentially eat crabs and shrimp. It is almost guaranteed that they will eat any that are in the tank.

Water Quality:
Eels are often tolerant of a wide range of water quality parameters. Below is a list of commonly measured water quality parameters and their acceptable ranges.

Specific Gravity – 1.025
Temperature – 23-30 C
Dissolved O2 – >5 mg/L
pH – 6.5 – 9.0
Ammonia (NH3) – < 1.1 mg/L
Nitrite (NH4) – < 86 mg/L
Calcium – 100 – 150 mg/L

Eels are carnivorous. Eels are voracious eaters, and often adapt quickly to new food types. Any fishmeal-based food, even frozen, will be gobbled up by most eels. Although non-fishmeal diets have been explored, due to their lower costs, long-term nutritional deficiencies are a possibility. Although the exact dietary protein requirements for eels has not been established, a general range of 35 – 45% is recommended for most freshwater species. A word of warning—do not attempt to hand feed your eel. Eels have poor eyesight and an excellent sense of smell. Many become extremely excited by the smell of food, and eel bites are painful and often become infected.

Feedings should be done 1 -2 times a week, and only as much as they will readily consume. Extra food in aquatic systems will convert to toxic byproducts and give extra nutrition to the microflora in the tank, potentially causing excessive algae growth. It is not uncommon for eels to enter periods of “hibernation”, entering hiding and refusing food for a few weeks. Eels, in particular, are extremely well adapted to periods of low food availability.

Like all fishes, most diseases in eels area due to issues with water quality. Treatment of the disease is often reliant upon correcting any imbalances in water quality, along with treatment of the affected individual(s). Systemic drugs can be added on to severely affected patients, in order to help directly treat the disease. Below, we’ve listed a few examples of different diseases. This list is by no means comprehensive, but does describe the clinical signs, diagnosis, and treatment of a variety of diseases that can affect your eel.

Edwardsiellosis is a potentially devastating bacterial disease caused by Edwardsiella tarda. Most commonly, it is reported in Japanese eels. Often, the first outward clinical signs of the disease are red, ulcerated lesions on the side and belly. These lesions, however, are formed by ulcerating abscesses of internal organs that have eroded through the musculature and skin. It primarily affects the kidneys and liver, but can spread to any organ system. Diagnosing the disease is often reliant upon clinical signs and culture of the bacteria from the wounds. The bacteria can infect humans and most commonly causes GI upset, but can cause severe disease in immunocompromised patients. Correcting water quality, along with systemic tetracycline antibiotics, is the treatment of choice.

“Water Mold” is a fungal infection of the skin that occurs in almost all freshwater species. Lesions present as a superficially cottony growth on the skin or gills. They start as white and change to red, brown, or green as they persist. They usually begin as small focal lesions, but can spread rapidly across the entire body and ulcerate deep into the body, if left untreated. Diagnosis relies on sampling the lesions, looking for fungal structures. Unfortunately, fungal infections are difficult to treat in fish, due to a lack of legally approved medications. Potassium permanganate pond treatments have seen some success, as well as salt dips. Malachite Green is an effective agent, but cannot be used in any “food fish”.