Diagnosing and Treating Dropsy in Aquarium Fish

Fish dropsy
Clintus/flickr by CC 2.0

Dropsy is an old medical term for a medical condition that today would be more likely called edema—the swelling of soft tissues in a body cavity, such as the abdomen, due to an accumulation of water and other fluids. The English term derives from the Middle English word dropesie, from the Old French word hydropse, from the Greek word hydrops, which is itself a derivation of hydro, meaning water.  

Dropsy in Aquarium Fish

Although the term dropsy is rarely heard in human medical science these days, it is still used to describe a particular health issue with aquarium fish. Fish suffering from dropsy often have hugely swollen bellies, and the continuing use of the term probably has to do with how it so accurately depicts the visual symptom: the belly drops down. Sometimes the condition is also known as bloat

Dropsy in fish is actually a cluster of symptoms caused by an infection from a bacteria commonly present in all aquariums. Consequently, any fish may harbor the dropsy-causing bacteria, but healthy fish rarely fall prey to the disease. Fish are only susceptible when their immune system has been compromised by some other stress factor. If all the fish in the tank are under stress, it’s quite common for the entire tank to become infected, but it is also possible for only one or two fish to fall ill, especially when prompt action is taken to prevent the spread of the bacteria.

As the infection progresses, skin lesions may appear, the belly fills with fluids and becomes swollen, internal organs are damaged, and ultimately the fish will die. Even with prompt treatment, the mortality rate is high. Successful treatment is very unlikely unless a fish is diagnosed in the early stages of the infection.

Symptoms

Strictly speaking, dropsy is not a disease but is instead a cluster of symptoms of an underlying infection. Symptoms of the underlying bacterial infection can vary widely. Some fish will have the classic swollen belly, others display skin lesions, while still others show few symptoms at all. This variability is what makes diagnosis difficult. In most cases, a number of symptoms are observed, both physical and behavioral. These may include:

  • Grossly swollen belly
  • Scales that stand out with a pinecone-like appearance
  • Eyes that bulge
  • Gills that are pale
  • The anus that becomes red and swollen
  • Feces that is pale and stringy
  • Ulcer on the body, along the lateral line
  • A spine that is curved
  • Fins clamped together
  • General lethargy
  • Refusal to eat
  • Swimming near the surface. 

These symptoms occur progressively as the disease advances. Internal organs are affected, most notably the liver and kidneys. Anemia occurs, causing the gills to lose their normal red color. As the abdomen fills with fluid, organs are pushed aside, sometimes causing the spine to curve. Scales protrude from the body, giving the appearance of a pine cone. This symptom is a classic indication of a severe infection.

Cause

The agent that causes the symptoms of dropsy is usually the very common Aeromonas bacteria, one of several so-called gram-negative bacteria present in most aquarium habitats.

The bacteria is known as gram-negative because it does react to a common gram-staining identification procedure.

The bacteria will only lead to the serious infection in a fish that already has a compromised immune system. This can happen as the result of stress from a number of factors, such as the following:

  • Poor water quality
  • Ammonia or nitrite spikes
  • A large drop in water temperature
  • Stress from transportation
  • Improper nutrition
  • Aggressive tankmates
  • Other diseases

Generally, a single or short-term exposure to stress will not compromise the ability of the fish to fight infection. In most cases, the stress exposure must be present for an extended period of time, or several stress factors must occur in rapid succession, in order to affect the fish's immune system. 

Treatment

The infection causing dropsy is not easily cured.

Some experts recommend that all affected fish be euthanized to prevent the spread of the infection to healthy fish. However, if the infection is detected early, it is possible to save affected fish. Treatment is geared towards correcting the underlying problem and providing supportive care to the sick fish: 

  • Move the sick fish to a "hospital tank"
  • Add salt to the hospital tank—1 teaspoon per gallon
  • Feed the fish fresh, high-quality foods
  • Treat with antibiotics

It is important to move the infected fish to another tank to separate them from the remaining healthy fish. While the affected fish is quarantined, perform a water change on the original tank and monitor the remaining fish closely for appearance of symptoms.

Salt should be added to the hospital tank, at the ratio of one teaspoon per gallon of water. Keep the hospital tank scrupulously clean, and perform weekly partial water changes.

Provide the ill fish with a variety of fresh, high-quality food. Often this is enough to resolve the infection in cases that are not too far advanced. Keep the fish under observation for several weeks after the symptoms disappear.

Antibiotics should be used if the fish does not immediately respond. A broad-spectrum antibiotic specifically formulated for gram-negative bacteria is recommended, such as Maracyn-Two. A ten-day course is ideal for ensuring the infection is eradicated, but you should always follow manufacturer's directions for duration and dosage.

Prevention

As with many diseases, prevention is the best cure. Almost all the factors that stress fish enough to make them susceptible to infection can be prevented. Because poor water quality is the most common root cause of stress, tank maintenance is critical. Factors to keep in mind include:

  • Perform regular water changes
  • Keep the tank clean
  • Clean the filter regularly
  • Avoid overcrowding the tank
  • Do not overfeed fish
  • Use flake foods within one month of opening their packages
  • Vary the diet

If the tank is well maintained and the fish are fed a healthy diet, outbreaks of infections causing dropsy are unlikely.